return zero from dec_to_float when the mantissa is zero

[Ramya-9353]

Repro: parse("0e400") with default options returns a double that is NaN, and serialize of that value emits null, so the number changes JSON type on a round trip. 0e309, -0e400, 0.0e400 and 0E+400 behave the same. The same literals give 0 and -0 under number_precision::precise, and 1e400 gives infinity as intended.
Cause: dec_to_float scales the mantissa with x * pow10(e), and pow10 returns infinity for any exponent above 308. With a zero mantissa that product is 0 * inf, which is NaN.
Fix: return the already signed zero before the scaling when the mantissa is zero, so -0e400 gives -0.0 like the precise path. The two added grind_double cases fail on develop (13 assertions across the split points) and pass with the change.

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