Fix: crash on empty theme file settings (Mudlet#7560)

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#### Brief overview of PR changes/additions
Fix crash on empty theme file settings by assuming the default theme.
#### Motivation for adding to Mudlet
Closes Mudlet#7557
#### Other info (issues closed, discussion etc)
An alternative to Mudlet#7551 - this
solution is more straightforward.

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Co-authored-by: Vadim Peretokin <[email protected]>

src/XMLimport.cpp

@@ -786,6 +786,10 @@ void XMLimport::readHost(Host* pHost)
     pHost->mMapperShowRoomBorders = readDefaultTrueBool(qsl("mMapperShowRoomBorders"));
     pHost->mEditorTheme = attributes().value(QLatin1String("mEditorTheme")).toString();
     pHost->mEditorThemeFile = attributes().value(QLatin1String("mEditorThemeFile")).toString();
+    if (pHost->mEditorTheme.isEmpty() || pHost->mEditorThemeFile.isEmpty()) {
+        pHost->mEditorTheme = qsl("Mudlet");
+        pHost->mEditorThemeFile = qsl("Mudlet.tmTheme");
+    }
     pHost->mThemePreviewItemID = attributes().value(QLatin1String("mThemePreviewItemID")).toInt();
     pHost->mThemePreviewType = attributes().value(QLatin1String("mThemePreviewType")).toString();
     pHost->setHaveColorSpaceId(attributes().value(QLatin1String("mSGRCodeHasColSpaceId")).toString() == QLatin1String("yes"));