gstvalue: Transform an empty array to an empty string

A null-pointer is a valid value for an empty GValueArray. An empty array is a default value for an array property. Since _gst_value_transform_g_value_array_string doesn't check if the array is empty, one that calls g_param_spec_get_default_value and tries to convert the returned value to a string ends with a segmentation fault. For example, libglibmm_generate_extra_defs-2.68.so does this.

Fixes #2874