libvaladoc/girimporter: Fallback to "name" for callback
libvaladoc/girimporter: Fallback to "name" for callback

This caused criticals like:

  valadoc_importer_gir_documentation_importer_attach_comment: assertion 'cname != NULL' failed

frida/vala,frida/vala,GNOME/vala,frida/vala,frida/vala,GNOME/vala,GNOME/vala,frida/vala,GNOME/vala