fix: allow null data for empty binary and string views during append

[fornwall]

ArrowBufferView and ArrowStringView allow a null data pointer when their size is zero:

If size_bytes is 0, this value may be NULL.

Appending these views to binary-view or string-view arrays currently passes the null pointer to memcpy, which is undefined behaviour even for a zero-byte copy as flagged by UBSan.

Skip the inline copy for empty values to fix that.

[paleolimbot]

Thank you!

[codecov-commenter]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 78.19%. Comparing base (33af79a) to head (4e4d712).

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #940      +/-   ##
==========================================
+ Coverage   78.16%   78.19%   +0.02%     
==========================================
  Files         106      106              
  Lines       16842    16843       +1     
  Branches     1984     1985       +1     
==========================================
+ Hits        13165    13170       +5     
+ Misses       2443     2439       -4     
  Partials     1234     1234              

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