Coding Standards: Remove unnecessary conditional in WP_List_Table::print_column_headers().

This commit removes an unnecessary `empty()` check, where the `$class` variable is primed at the top of the `foreach` loop and will always be a non-empty array.

To maintain readability of the source of the `$class` attribute without the unnecessary `empty()` check, the resulting variable has been renamed to `$class_attr`, instead of reusing the array.

Follow-up to [8923], [15491].

Props justlevine.
See #52217.

git-svn-id: https://develop.svn.wordpress.org/trunk@60132 602fd350-edb4-49c9-b593-d223f7449a82

Author: SergeyBiryukov

src/wp-admin/includes/class-wp-list-table.php

@@ -1524,15 +1524,12 @@ public function print_column_headers( $with_id = true ) {
 				);
 			}
 
-			$tag   = ( 'cb' === $column_key ) ? 'td' : 'th';
-			$scope = ( 'th' === $tag ) ? 'scope="col"' : '';
-			$id    = $with_id ? "id='$column_key'" : '';
+			$tag        = ( 'cb' === $column_key ) ? 'td' : 'th';
+			$scope      = ( 'th' === $tag ) ? 'scope="col"' : '';
+			$id         = $with_id ? "id='$column_key'" : '';
+			$class_attr = "class='" . implode( ' ', $class ) . "'";
 
-			if ( ! empty( $class ) ) {
-				$class = "class='" . implode( ' ', $class ) . "'";
-			}
-
-			echo "<$tag $scope $id $class $aria_sort_attr $abbr_attr>$column_display_name</$tag>";
+			echo "<$tag $scope $id $class_attr $aria_sort_attr $abbr_attr>$column_display_name</$tag>";
 		}
 	}