Skip null attribute during DB update

[tracefinder]

This PR is based on a patch from https://github.com/tpm2-software/tpm2-pkcs11/issues/845 and fixes the issue when there is no CKA_ALLOWED_MECHANISMS attribute during DB migration

[codecov[bot]]

Codecov Report

Merging #849 (9873237) into master (1b3aab9) will decrease coverage by 0.01%. The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master     #849      +/-   ##
==========================================
- Coverage   72.43%   72.42%   -0.01%     
==========================================
  Files          34       34              
  Lines        9773     9774       +1     
==========================================
  Hits         7079     7079              
- Misses       2694     2695       +1     

Files	Coverage Δ
src/lib/db.c	67.41% <0.00%> (-0.06%)	:arrow_down:

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[tracefinder]

I guess opensuse-leap has the same issue as in https://github.com/tpm2-software/tpm2-pkcs11/pull/808#issuecomment-1398563650

[tracefinder]

@williamcroberts sorry for bothering you. Could you take a look at this PR? It's not big.

[idlesign]

Link to #858