[processing][SAGA] Fix default output format for geometry-less tables

(cherry picked from commit ef2fff0)

Author: nyalldawson

python/plugins/processing/algs/saga/SagaAlgorithmProvider.py

@@ -140,6 +140,9 @@ def supportedOutputRasterLayerExtensions(self):
     def supportedOutputVectorLayerExtensions(self):
         return ['shp', 'dbf']
 
+    def supportedOutputTableExtensions(self):
+        return ['dbf']
+
     def supportsNonFileBasedOutput(self):
         """
         SAGA Provider doesn't support non file based outputs