Enhance parameter defaults in various datasets (strict validation)

(cherry picked from commit 30fb9b6)

# Conflicts:
#	plotpy/styles/image.py
#	plotpy/styles/label.py

Author: PierreRaybaut

plotpy/styles/image.py

@@ -433,7 +433,7 @@ class MaskedImageParamMixin(DataSet):
     """Mixin for masked image parameters."""
 
     g_mask = BeginGroup(_("Mask"))
-    filling_value = FloatItem(_("Filling value"), default=0.0)
+    filling_value = FloatItem(_("Filling value"), allow_none=True)
     show_mask = BoolItem(_("Show image mask"), default=False)
     alpha_masked = FloatItem(_("Masked area alpha"), default=0.7, min=0, max=1)
     alpha_unmasked = FloatItem(_("Unmasked area alpha"), default=0.0, min=0, max=1)