Fix extra blank lines and duplicate </details> in primer comment

- Remove extra blank line between header and first package section
- Remove blank lines inside <details> blocks for cleaner markdown
- Fix duplicate </details> when new_messages exist but missing_messages don't
  (checked `missing_messages` dict truthiness instead of `missing_messages["messages"]`)

Author: Pierre-Sassoulas

pylint/testutils/_primer/primer_compare_command.py

@@ -41,7 +41,7 @@ def _create_comment(self, comparator: Comparator) -> str:
     def _create_comment_for_package(
         self, package: str, new_messages: PackageData, missing_messages: PackageData
     ) -> str:
-        comment = f"\n\n**Effect on [{package}]({self.packages[package].url}):**\n"
+        comment = f"\n**Effect on [{package}]({self.packages[package].url}):**\n\n"
         # Create comment for new messages
         count = 1
         astroid_errors = 0
@@ -73,15 +73,15 @@ def _create_comment_for_package(
             )
         if new_non_astroid_messages:
             comment += (
-                "The following messages are now emitted:\n\n<details>\n\n"
+                "The following messages are now emitted:\n\n<details>\n"
                 + new_non_astroid_messages
-                + "\n</details>\n\n"
+                + "</details>\n\n"
             )
 
         # Create comment for missing messages
         count = 1
         if missing_messages["messages"]:
-            comment += "The following messages are no longer emitted:\n\n<details>\n\n"
+            comment += "The following messages are no longer emitted:\n\n<details>\n"
             print("No longer emitted:")
         for message in missing_messages["messages"]:
             comment += f"{count}) {message['symbol']}:\n*{message['message']}*\n"
@@ -99,8 +99,8 @@ def _create_comment_for_package(
             )
             count += 1
             print(message)
-        if missing_messages:
-            comment += "\n</details>\n\n"
+        if missing_messages["messages"]:
+            comment += "</details>\n\n"
         return comment
 
     def _truncate_comment(self, comment: str) -> str:

tests/testutils/_primer/batched_cases/expected.txt

@@ -1,19 +1,17 @@
 🤖 **Effect of this PR on checked open source code:** 🤖
 
 
-
 **Effect on [astroid](https://github.com/pylint-dev/astroid):**
+
 The following messages are no longer emitted:
 
 <details>
-
 1) locally-disabled:
 *Locally disabling redefined-builtin (W0622)*
 https://github.com/pylint-dev/astroid/blob/1234567890abcdef/astroid/__init__.py#L91
 2) unknown-option-value:
 *Unknown option value for 'disable', expected a valid pylint message and got 'Ellipsis'*
 https://github.com/pylint-dev/astroid/blob/1234567890abcdef/astroid/__init__.py#L91
-
 </details>
 
 *This comment was generated for commit v2.14.2*

tests/testutils/_primer/cases/message_changed/expected.txt

@@ -1,26 +1,22 @@
 🤖 **Effect of this PR on checked open source code:** 🤖
 
 
-
 **Effect on [astroid](https://github.com/pylint-dev/astroid):**
+
 The following messages are now emitted:
 
 <details>
-
 1) locally-disabled:
 *Locally disabling redefined-builtin [we added some text in the message] (W0622)*
 https://github.com/pylint-dev/astroid/blob/123456789abcdef/astroid/__init__.py#L91
-
 </details>
 
 The following messages are no longer emitted:
 
 <details>
-
 1) locally-disabled:
 *Locally disabling redefined-builtin (W0622)*
 https://github.com/pylint-dev/astroid/blob/123456789abcdef/astroid/__init__.py#L91
-
 </details>
 
 *This comment was generated for commit v2.14.2*

tests/testutils/_primer/cases/new_message/expected.txt

@@ -1,19 +1,14 @@
 🤖 **Effect of this PR on checked open source code:** 🤖
 
 
-
 **Effect on [astroid](https://github.com/pylint-dev/astroid):**
+
 The following messages are now emitted:
 
 <details>
-
 1) locally-disabled:
 *Locally disabling redefined-builtin [we added some text in the message] (W0622)*
 https://github.com/pylint-dev/astroid/blob/123456789abcdef/astroid/__init__.py#L91
-
-</details>
-
-
 </details>
 
 *This comment was generated for commit v2.14.2*

tests/testutils/_primer/cases/removed_message/expected.txt

@@ -1,16 +1,14 @@
 🤖 **Effect of this PR on checked open source code:** 🤖
 
 
-
 **Effect on [astroid](https://github.com/pylint-dev/astroid):**
+
 The following messages are no longer emitted:
 
 <details>
-
 1) unknown-option-value:
 *Unknown option value for 'disable', expected a valid pylint message and got 'Ellipsis'*
 https://github.com/pylint-dev/astroid/blob/123456789abcdef/astroid/__init__.py#L91
-
 </details>
 
 *This comment was generated for commit v2.14.2*