fix: zero dates come back as "undefined 00:00:00" with dateStrings

[NotAFlightRisk]

With dateStrings on, a zero DATETIME or TIMESTAMP read through execute() comes back as the string undefined 00:00:00, and a zero DATE as plain undefined. query() gives 0000-00-00 00:00:00 and 0000-00-00 for the same rows. Thats MySQL 9.7.2 here, I've not tried MariaDB. A typeCast function calling field.string() lands on it too, without dateStrings involved at all.

When every part of the value is zero the server sends a zero-length payload, so str never gets assigned and the midnight branch appends to undefined. test-datestrings-binary-zero-time.test.mts covers the neighbouring case but uses a real date at midnight, so it never reaches that path.

The !str check in readDateTime was only there to swalow the undefined (#4258), so it's looking for the zero date itself now. Read as Dates they're still Invalid Date and test-#1019.test.mts passes either way.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 92.09%. Comparing base (928c3c5) to head (87a6ede).

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #4491   +/-   ##
=======================================
  Coverage   92.09%   92.09%           
=======================================
  Files          92       92           
  Lines       15213    15214    +1     
  Branches     2086     2086           
=======================================
+ Hits        14011    14012    +1     
  Misses       1202     1202           

Flag	Coverage Δ
compression-0	91.65% <100.00%> (+<0.01%)	⬆️
compression-1	92.07% <100.00%> (+<0.01%)	⬆️
static-parser-0	90.86% <100.00%> (+<0.01%)	⬆️
static-parser-1	91.12% <100.00%> (+<0.01%)	⬆️
tls-0	91.64% <100.00%> (+<0.01%)	⬆️
tls-1	92.09% <100.00%> (+<0.01%)	⬆️

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