fix(generator): don't validate regex for non-required empty values

relates to STACKITSDK-125

Author: rubenhoenle

templates/python/model_generic.mustache

@@ -41,7 +41,7 @@ class {{classname}}({{#parent}}{{{.}}}{{/parent}}{{^parent}}BaseModel{{/parent}}
     def {{{name}}}_validate_regular_expression(cls, value):
         """Validates the regular expression"""
         {{^required}}
-        if value is None:
+        if value is None or value == "":
             return value
 
         {{/required}}