sqlite: check null returns from sqlite value functions

[ndossche]

sqlite3_column_text() can return nullptr on failure which was not handled. sqlite3_column_blob() can return nullptr for zero-length BLOBs, which is then passed to memcpy() which is UB. Avoid this by checking for a nullptr.

Note: this was found by a static-dynamic analyser I'm developing.

[nodejs-github-bot]

Review requested:

• @nodejs/sqlite

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 90.30%. Comparing base (a159b57) to head (91b4eb1).
⚠️ Report is 201 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #63288      +/-   ##
==========================================
+ Coverage   90.04%   90.30%   +0.26%     
==========================================
  Files         714      730      +16     
  Lines      225242   234802    +9560     
  Branches    42578    43956    +1378     
==========================================
+ Hits       202808   212027    +9219     
- Misses      14221    14494     +273     
- Partials     8213     8281      +68     

Files with missing lines	Coverage Δ
src/node_sqlite.cc	80.62% <ø> (-0.01%)	⬇️

... and 171 files with indirect coverage changes

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