Fix crash when painting on 3MF-loaded model

CURA-12528

Author: wawanbreton

UM/Mesh/MeshData.py

@@ -186,7 +186,7 @@ def getColors(self) -> numpy.ndarray:
         return self._colors
 
     def hasUVCoordinates(self) -> bool:
-        return self._uvs is not None
+        return self._uvs is not None and len(self._uvs) > 0
 
     def getFileName(self) -> Optional[str]:
         return self._file_name