Fix serialization of zero decimal values in IEEE754 mode

[AndreasMueck]

Summary

Fixes serialization of finite zero Edm.Decimal values when
IEEE754Compatible=true.

Previously, toJsonIeee754() evaluated log10(0.0), which resulted in an
infinite precision passed to sprintf() and caused an HTTP 500 response.

This change:

• returns "0" for zero decimal values;
• uses abs() for the precision calculation, avoiding log10() errors for
  negative finite decimals.

Reproduction

Persist an Edm.Decimal value of 0.00 and request it using:

Accept: application/json;IEEE754Compatible=true

Before this change, the response fails with:

The float INF is not representable as an int, cast occurred

Added a regression test for Decimal(0.0) in IEEE754 mode.

Verified with:

./vendor/bin/phpunit --filter test_decimal_zero_serializes_in_ieee754_mode