st/theme-node: Only try to parse non-empty inline styles

Otherwise cr_parser_new_from_buf() will fail creating a parser for a
buffer of length 0, resulting in further errors due to the parser being
NULL. This could happen when extensions were trying to set the style of
a widget to an empty string.

Fixes: https://gitlab.gnome.org/GNOME/gnome-shell/-/issues/4634
Part-of: <https://gitlab.gnome.org/GNOME/gnome-shell/-/merge_requests/2286>

src/st/st-theme-node.c

@@ -456,7 +456,7 @@ ensure_properties (StThemeNode *node)
       if (node->theme)
         properties = _st_theme_get_matched_properties (node->theme, node);
 
-      if (node->inline_style)
+      if (node->inline_style && *node->inline_style != '\0')
         {
           CRDeclaration *cur_decl;