solved Devtools show null instead of NaN #1781

[KirtiGautam620]

Fix: Devtools show null instead of NaN (#1781)

Summary

This PR fixes an issue where TanStack Form Devtools incorrectly showed null
for fields whose value was explicitly set to Number.NaN. This caused numeric
fields that were intentionally initialized or updated to NaN to appear as
null or even disappear in the Devtools UI, making debugging confusing.

The browser correctly treats <input type="number"> with value "" as "not
set", so many users (including the reporter) map NaN -> "" when binding input
values. However, inside the form state, TanStack Form should preserve the actual
value NaN. Devtools was stringifying the value and normalizing it to null,
leading to incorrect visualization.

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Problem

When a field value is Number.NaN, Devtools displays:

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[KirtiGautam620]

@tannerlinsley hi! i have worked and fixed this issue. kindly have a look over it and let me know what i can change. Thanks.