Fix MergeYaml Document.End fallback inserting extra blank lines (#7311)

The Document.End prefix fallback introduced in #7291 would copy
trailing whitespace (e.g. a bare newline) as a "comment" prefix
onto newly merged entries, causing an extra blank line in the output.

Only use the Document.End prefix when it actually contains a YAML
comment (has a `#` character). Plain trailing whitespace should not
be treated as a comment to preserve.

Fixes the `SeparateApplicationYamlByProfileTest.mergeIntoExistingProfileFilePreservesFormat`
failure in openrewrite/rewrite-spring.

Author: timtebeek

rewrite-yaml/src/main/java/org/openrewrite/yaml/MergeYamlVisitor.java

@@ -256,8 +256,13 @@ private Yaml.Mapping mergeMapping(Yaml.Mapping m1, Yaml.Mapping m2, P p, Cursor
                         if (docCursor.getValue() instanceof Yaml.Document) {
                             Yaml.Document doc = docCursor.getValue();
                             if (!preserveDocumentSeparator(doc)) {
-                                comment = doc.getEnd().getPrefix();
-                                c = docCursor;
+                                String endPrefix = doc.getEnd().getPrefix();
+                                // Only use Document.End prefix if it contains a comment;
+                                // plain trailing whitespace should not be copied as a comment
+                                if (endPrefix != null && endPrefix.contains("#")) {
+                                    comment = endPrefix;
+                                    c = docCursor;
+                                }
                             }
                         }
                     }