Core: fix IDocuments aboutToSave and saved filePath

The function emiting those signals can be called with an empty file
path. Use the document file path in this case like in
TextDocument::saveImpl.

Change-Id: I9e3381999a25c49df1d5db060ef5467b12220ad4
Reviewed-by: Christian Kandeler <christian.kandeler@qt.io>

src/plugins/coreplugin/idocument.cpp

@@ -343,10 +343,10 @@ IDocument::OpenResult IDocument::open(QString *errorString, const Utils::FilePat
 */
 bool IDocument::save(QString *errorString, const Utils::FilePath &filePath, bool autoSave)
 {
-    emit aboutToSave(filePath, autoSave);
+    emit aboutToSave(filePath.isEmpty() ? this->filePath() : filePath, autoSave);
     const bool success = saveImpl(errorString, filePath, autoSave);
     if (success)
-        emit saved(filePath, autoSave);
+        emit saved(filePath.isEmpty() ? this->filePath() : filePath, autoSave);
     return success;
 }