fix: encode NaN/Inf floats as null in JSON encoder

[toller892]

Problem

The JSON encoder encodes NaN, +Inf, and -Inf as quoted strings ("NaN", "+Inf", "-Inf"), producing invalid JSON. This is inconsistent with Go's encoding/json, which returns an UnsupportedValueError for these values.

// Current behavior:
enc.AddFloat64("k", math.NaN())  // outputs: "k":"NaN"   (string, not a number)
enc.AddFloat64("k", math.Inf(1)) // outputs: "k":"+Inf"  (string, not a number)

This means:

1. zap.Reflect() with a struct containing NaN returns an error, but enc.AddFloat64() silently produces a string — inconsistent behavior within zap itself.
2. The output is not valid JSON — "NaN" is a string, not a numeric value.
3. Consumers parsing zap's JSON output cannot distinguish between a legitimate string field and a mis-encoded float.

Fix

Encode NaN, +Inf, and -Inf as null in the JSON encoder. null is valid JSON and the most common representation for non-finite numbers in JSON serializers.

// After fix:
enc.AddFloat64("k", math.NaN())  // outputs: "k":null
enc.AddFloat64("k", math.Inf(1)) // outputs: "k":null
enc.AddFloat64("k", math.Inf(-1))// outputs: "k":null

Changes

• zapcore/json_encoder.go: Change appendFloat() to emit null instead of "NaN"/"+Inf"/"-Inf"
• zapcore/json_encoder_impl_test.go: Update test expectations for NaN/Inf float64 and float32 cases

Fixes #690

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