Fix use of 'is' operator for comparison

The 'is' operator is not meant to be used for comparisons. It currently working is an implementation detail of CPython.
CPython 3.8 has added a SyntaxWarning for this.
diff --git a/scripts/parameter_validation_generator.py b/scripts/parameter_validation_generator.py
index eb6f97d..83b00cf 100644
--- a/scripts/parameter_validation_generator.py
+++ b/scripts/parameter_validation_generator.py
@@ -361,10 +361,10 @@
                 struct_validation_source = self.ScrubStructCode(expr)
                 pnext_case += '%s' % struct_validation_source
                 pnext_case += '        } break;\n'
-                if protect is not '':
+                if protect:
                     pnext_case += '#endif // %s\n' % protect
                 # Skip functions containing no validation
-                if struct_validation_source != '':
+                if struct_validation_source:
                     pnext_handler += pnext_case;
             pnext_handler += '        default:\n'
             pnext_handler += '            skip = false;\n'