ResultSetRecordSet Breaks if First Row Decimal Value is NULL

Changes in NIFI-7369 caused a breaking change in how Decimal values are converted into schemas in ResultSetRecordSet.

The code tries to read the scale and precision values, but reads them from the value in the first row returned, rather than from the ResultSetMetadata.  If the value in the first row is NULL, than it's scale and precision cannot be read and schema creation fails.