The current values_cmp() method will segfault if an object contains a NULL
GeglColor value. I fixed it in commit c0477bcb0 in GEGL, but since the GEGL
release is already done, I add this special-casing on GIMP side.

To be removed when we release and depend on GEGL 0.4.50.

The crash was happening when activating the text tool and raised in a comment of
another report: #10813 (comment 2009702)