rex4j produces empty strings instead of nulls for empty result

[eemhu]

Describe the bug

rex4j empty rows should be null instead of empty string

Expected behavior

These empty rows should be of a standardized null value instead of an empty string so isnull() and isnotnull() will pick up on these values.

How to reproduce

Use rex4j with a non-matching capture group which will result in empty rows in the result column.

Screenshots

Software version

5.1.1

Desktop (please complete the following information if relevant):

• OS:
• Browser:
• Version:

Additional context

rex already behaves like this, rex4j should do the same.

[eemhu]

internal PR submitted and up for review

[51-code]

internal PR reviewed