[PD] code improvement for FeaturePad

as noted by @chennes in PR #5164, we should follow the C++ style guidelines:
https://isocpp.github.io/CppCoreGuidelines/CppCoreGuidelines#Res-if

src/Mod/PartDesign/App/FeaturePad.cpp

@@ -147,7 +147,7 @@ App::DocumentObjectExecReturn *Pad::execute(void)
         Base::Vector3d paddingDirection;
         
         if (!UseCustomVector.getValue()) {
-            if (ReferenceAxis.getValue() == nullptr) {
+            if (!ReferenceAxis.getValue()) {
                 // use sketch's normal vector for direction
                 paddingDirection = SketchVector;
                 AlongSketchNormal.setReadOnly(true);