Fix empty filters in project files

Fix loading of project files with empty filter values. If an empty
filter value was saved in a project file, it did not show up in the UI
but was partially added to the query anyway. This lead to an invalid
query with no results.

Also fix a possible loophole which at least in theory could lead to an
empty filter value being stored in the project file.

See issue #2288.

src/MainWindow.cpp

@@ -2320,7 +2320,9 @@ static void loadBrowseDataTableSettings(BrowseDataTableSettings& settings, QXmlS
             while(xml.readNext() != QXmlStreamReader::EndElement && xml.name() != "filter_values") {
                 if (xml.name() == "column") {
                     size_t index = xml.attributes().value("index").toUInt();
-                    settings.filterValues[index] = xml.attributes().value("value").toString();
+                    QString value = xml.attributes().value("value").toString();
+                    if(!value.isEmpty())
+                        settings.filterValues[index] = value;
                     xml.skipCurrentElement();
                 }
             }

src/TableBrowser.cpp

@@ -525,9 +525,10 @@ void TableBrowser::updateFilter(size_t column, const QString& value)
 
     // Save the new filter settings
     BrowseDataTableSettings& settings = m_settings[currentlyBrowsedTableName()];
-    if(value.isEmpty() && settings.filterValues.erase(column) > 0)
+    if(value.isEmpty())
     {
-        emit projectModified();
+        if(settings.filterValues.erase(column) > 0)
+            emit projectModified();
     } else {
         if (settings.filterValues[column] != value) {
             settings.filterValues[column] = value;