Fix set diff fallback for Optional non-Computed fields without Default

[wlami]

Summary

When a TypeSet block contains an Optional, non-Computed field without a Default that the user doesn't specify, SDKv2 fills in the zero value ("" for strings) in the planned state. The bridge's validInputsFromPlan function was failing to match these elements because null != "".

This caused noisy whole-set UPDATE diffs instead of precise element-level diffs on resources like google_compute_url_map where host_rule has an optional description field.

Fix

The fix treats null input as equal to the zero value for Optional non-Computed fields, allowing matchPlanElementsToInputs to succeed and produce correct element-level diffs.

Added isZeroValue helper that checks if a property value is the zero value for its schema type (empty string, false, 0, empty array/map).

Testing

• Added TestSDKv2DetailedDiffSetBlockOptionalUnspecified with 40 test cases covering add/remove/shuffle scenarios
• All existing set diff tests pass

Before/After

Before: Adding one host rule to a URL map with 40 rules shows:

~ hostRules: UPDATE

After: Shows precise element-level diff:

~ hostRules: [
    + [5]: { hosts: ["new.example.com"], pathMatcher: "pm-new" }
  ]

Fixes #3324, replaces PR #3325

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 69.77%. Comparing base (9ce27ac) to head (3ea346b).
⚠️ Report is 52 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #3338      +/-   ##
==========================================
- Coverage   69.77%   69.77%   -0.01%     
==========================================
  Files         339      339              
  Lines       36750    36753       +3     
==========================================
+ Hits        25644    25645       +1     
- Misses       9360     9363       +3     
+ Partials     1746     1745       -1     

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