Remove redundant null check

InstanceOf already returns false for null.

Change-Id: Idf5a99252c8b19f08dbda08a1cf16d61d7d56934
Reviewed-on: https://git.eclipse.org/r/c/platform/eclipse.platform.text/+/187489
Tested-by: Alexander Kurtakov <akurtako@redhat.com>
Reviewed-by: Alexander Kurtakov <akurtako@redhat.com>