DATAJDBC-355 - Remove unnecessary null check.

[benelog]

The following logic to check null value seems unnecessary.

	@ReadingConverter
	enum BigDecimalToString implements Converter<BigDecimal, String> {

		INSTANCE;

		@Override
		public String convert(@Nullable BigDecimal source) {

			if (source == null) {
				return null;
			}

			return source.toString();
		}
	}

The source parameter must not be null according to Javadoc of Converter.

https://docs.spring.io/spring/docs/current/javadoc-api/org/springframework/core/convert/converter/Converter.html

[schauder]

That's merged.
Thanks.