odb: do not use "blank" substitute for NULL

When various *object_info() functions are given an extended object
info structure as NULL by a caller that does not want any details,
the code uses a file-scope static blank_oi and passes it down to
the helper functions they use, to avoid handling NULL specifically.

The ps/object-read-stream topic graduated to 'master' recently
however had a bug that assumed that two identically named file-scope
static variables in two functions are the same, which of course is
not the case.  This made "git commit" take 0.38 seconds to 1508
seconds in some case, as reported by Aaron Plattner here:

  https://lore.kernel.org/git/f4ba7e89-4717-4b36-921f-56537131fd69@nvidia.com/

We _could_ move the blank_oi variable to the global scope in common
section to fix this regression, but explicitly handling the NULL is
a much safer fix.  It would also reduce the chance of errors that
somebody accidentally writes into blank_oi, making its contents
dirty, which potentially will make subsequent calls into the
function misbehave.  By explicitly handling NULL input, we no longer
have to worry about it.

Reported-by: Aaron Plattner <aplattner@nvidia.com>
Signed-off-by: Junio C Hamano <gitster@pobox.com>

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