gles3: Ensure missing_extensions is always populated

This appears to be the only explanation for `init_secondary_gpu_data_gpu`
crashing in `g_strjoinv`, but I don't know the exact conditions causing
`glGetString (GL_EXTENSIONS)` to return NULL.

https://launchpad.net/bugs/1994011

Part-of: <https://gitlab.gnome.org/GNOME/mutter/-/merge_requests/2674>

src/backends/meta-gles3.c

@@ -127,7 +127,7 @@ meta_gles3_has_extensions (MetaGles3    *gles3,
   if (!extensions_str)
     {
       g_warning ("Failed to get string: %s", get_gl_error_str (glGetError ()));
-      return FALSE;
+      extensions_str = "";
     }
 
   va_start (var_args, first_extension);