shell-extensions: Disable the plugin if we can't get the current shell version

This may explain the thousands of critical warnings and memory explosion in
#486

Relevant parts of the log:

    json_object_has_member: assertion 'member_name != NULL' failed
    g_strsplit: assertion 'string != NULL' failed
    g_strv_length: assertion 'str_array != NULL' failed
1 job for <span class="ref-name">wip/hughsie/shell-no-version</span> in 19 minutes and 40 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Build
passed #267702
build-gnome-software

00:19:40