app: Don't crash if no help handler is registered

g_app_info_get_default_for_uri_scheme() is documented to return NULL on
error, so handle that case.

Fixes #99
1 job for fix-help-crash in 4 minutes and 6 seconds (queued for 2 seconds)
Status Job ID Name Coverage
  Test
passed #593454
build-fedora

00:04:06