Do not save .qgd file alongside .qgs when it's not used

python/core/qgsauxiliarystorage.sip.in

@@ -345,6 +345,15 @@ Duplicates a table and its content.
     static QString extension();
 %Docstring
 Returns the extension used for auxiliary databases.
+%End
+
+    static bool exists( const QgsProject &project );
+%Docstring
+Returns true if the auxiliary database yet exists for a project, false otherwise.
+
+:param project: The project for which the database is checked
+
+.. versionadded:: 3.2
 %End
 
 };

src/core/qgsauxiliarystorage.cpp

@@ -647,6 +647,12 @@ QString QgsAuxiliaryStorage::extension()
   return AS_EXTENSION;
 }
 
+bool QgsAuxiliaryStorage::exists( const QgsProject &project )
+{
+  const QFileInfo fileinfo( filenameForProject( project ) );
+  return fileinfo.exists() && fileinfo.isFile();
+}
+
 bool QgsAuxiliaryStorage::exec( const QString &sql, sqlite3 *handler )
 {
   bool rc = false;

src/core/qgsauxiliarystorage.h

@@ -377,6 +377,15 @@ class CORE_EXPORT QgsAuxiliaryStorage
      */
     static QString extension();
 
+    /**
+     * Returns true if the auxiliary database yet exists for a project, false otherwise.
+     *
+     * \param project The project for which the database is checked
+     *
+     * \since QGIS 3.2
+     */
+    static bool exists( const QgsProject &project );
+
   private:
     spatialite_database_unique_ptr open( const QString &filename = QString() );
     spatialite_database_unique_ptr open( const QgsProject &project );

src/core/qgsproject.cpp

@@ -2505,6 +2505,7 @@ void QgsProject::setTrustLayerMetadata( bool trust )
 bool QgsProject::saveAuxiliaryStorage( const QString &filename )
 {
   const QMap<QString, QgsMapLayer *> layers = mapLayers();
+  bool empty = true;
   for ( auto it = layers.constBegin(); it != layers.constEnd(); ++it )
   {
     if ( it.value()->type() != QgsMapLayer::VectorLayer )
@@ -2514,10 +2515,15 @@ bool QgsProject::saveAuxiliaryStorage( const QString &filename )
     if ( vl && vl->auxiliaryLayer() )
     {
       vl->auxiliaryLayer()->save();
+      empty &= vl->auxiliaryLayer()->auxiliaryFields().isEmpty();
     }
   }
 
-  if ( !filename.isEmpty() )
+  if ( !mAuxiliaryStorage->exists( *this ) && filename.isEmpty() && empty )
+  {
+    return true; // it's not an error
+  }
+  else if ( !filename.isEmpty() )
   {
     return mAuxiliaryStorage->saveAs( filename );
   }