notes: clean up confusing NULL checks in init_notes()

notes: clean up confusing NULL checks in init_notes()

Coverity complains that we check whether "notes_ref" is NULL, but it was
already implied to be non-NULL earlier in the function. And this is
true; since b9342b3fd63 (refs: add array of ref namespaces, 2022-08-05),
we call xstrdup(notes_ref) unconditionally, which would segfault if it
was NULL.

But that commit is actually doing the right thing. Even if NULL is
passed into the function, we'll use default_notes_ref() as a fallback,
which will never return NULL (it tries a few options, but its last
resort is a string literal). Ironically, the "!notes_ref" check was
added by the same commit that added the fallback: 709f79b0894 (Notes
API: init_notes(): Initialize the notes tree from the given notes ref,
2010-02-13). So this check never did anything.

Signed-off-by: Jeff King <peff@peff.net>
Signed-off-by: Junio C Hamano <gitster@pobox.com>