fix: skip empty array values in stringify

[deepview-autofix]

Empty array values caused malformed query strings with leading or trailing & separators because the separator was appended unconditionally for every key while the inner loop produced no output. For example, stringify({a: [], b: "1"}) produced "&b=1" and stringify({a: "1", b: []}) produced "a=1&".

Skip empty arrays entirely and only emit the separator when the current key actually contributes a key=value pair, matching the behavior of the native querystring module.

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.31%. Comparing base (9dcbaf2) to head (9d137ff).
⚠️ Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main      #91      +/-   ##
==========================================
+ Coverage   68.97%   69.31%   +0.34%     
==========================================
  Files           7        7              
  Lines         448      453       +5     
  Branches       72       74       +2     
==========================================
+ Hits          309      314       +5     
  Misses        136      136              
  Partials        3        3              

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[anonrig]

How does node:query string behave?

[ChALkeR]

@anonrig correctly.

> querystring.stringify({a:[],b:"1"})
'b=1'