patch 8.1.0011: maparg() and mapcheck() confuse empty and non-existing

Problem:    maparg() and mapcheck() confuse empty and non-existing.
Solution:   Return <Nop> for an existing non-empty mapping. (closes #2940)

Author: brammool

src/evalfunc.c

@@ -7383,7 +7383,12 @@ get_maparg(typval_T *argvars, typval_T *rettv, int exact)
     {
 	/* Return a string. */
 	if (rhs != NULL)
-	    rettv->vval.v_string = str2special_save(rhs, FALSE);
+	{
+	    if (*rhs == NUL)
+		rettv->vval.v_string = vim_strsave((char_u *)"<Nop>");
+	    else
+		rettv->vval.v_string = str2special_save(rhs, FALSE);
+	}
 
     }
     else if (rettv_dict_alloc(rettv) != FAIL && rhs != NULL)

src/testdir/test_maparg.vim

@@ -29,9 +29,13 @@ function Test_maparg()
         \ maparg('foo', '', 0, 1))
 
   map abc x<char-114>x
-  call assert_equal(maparg('abc'), "xrx")
+  call assert_equal("xrx", maparg('abc'))
   map abc y<S-char-114>y
-  call assert_equal(maparg('abc'), "yRy")
+  call assert_equal("yRy", maparg('abc'))
+
+  map abc <Nop>
+  call assert_equal("<Nop>", maparg('abc'))
+  unmap abc
 endfunction
 
 function Test_range_map()

src/version.c

@@ -761,6 +761,8 @@ static char *(features[]) =
 
 static int included_patches[] =
 {   /* Add new patch number below this line */
+/**/
+    11,
 /**/
     10,
 /**/