Fix `bigDecimalDecoderScala` for Scala 2.12

[guizmaii]

In Scala 2.12, the javaBigDecimal2bigDecimal is not checking for null before the call apply while in Scala 2.13+, if does check, see:

• Scala 2.12: https://github.com/scala/scala/blob/2.12.x/src/library/scala/math/BigDecimal.scala#L347
• Scala 2.13: https://github.com/scala/scala/blob/2.13.x/src/library/scala/math/BigDecimal.scala#L307