hotdoc: move None check to caller of set_arg_value

Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

mesonbuild/modules/hotdoc.py

@@ -99,15 +99,13 @@ class HotdocTargetBuilder:
             argname = option.strip("-").replace("-", "_")
 
         value = self.kwargs.pop(argname)  # type: ignore[misc]
-        if value is not None and value_processor:
+        if value is None:
+            return
+        if value_processor:
             value = value_processor(value)
-
         self.set_arg_value(option, value)
 
     def set_arg_value(self, option: str, value: TYPE_var) -> None:
-        if value is None:
-            return
-
         if isinstance(value, bool):
             if value:
                 self.cmd.append(option)