Revert "select: autogenerator: No need to NULL guard lwc_string_unref"

This reverts commit c812359.

Author: boxerab

src/select/autogenerated_propset.h

@@ -127,7 +127,8 @@ static inline css_error set_background_image(css_computed_style *style, uint8_t
 		style->i.background_image = NULL;
 	}
 
-	lwc_string_unref(old_string);
+	if (old_string != NULL)
+		lwc_string_unref(old_string);
 
 	return CSS_OK;
 }
@@ -1439,7 +1440,8 @@ static inline css_error set_list_style_image(css_computed_style *style, uint8_t
 		style->i.list_style_image = NULL;
 	}
 
-	lwc_string_unref(old_string);
+	if (old_string != NULL)
+		lwc_string_unref(old_string);
 
 	return CSS_OK;
 }

src/select/select_generator.py

@@ -588,7 +588,10 @@ def make_propset_h(self):
                     t.indent(-1)
                     t.append('}')
                     t.append()
+                    t.append('if ({} != NULL)'.format(old_n))
+                    t.indent(1)
                     t.append('lwc_string_unref({});'.format(old_n))
+                    t.indent(-1)
 
                 elif v.name == 'string_arr' or v.name == 'counter_arr':
                     iter_var = 's' if v.name == 'string_arr' else 'c'