Consolidate whitespace handling

## What?

We have places in Graylog where we check/replace whitespace. In some of the occurrences (see link below) we check for a simple "space" char - which is not, what Java suggests as whitespace. If we allow a "space" in field names at some point, we should consolidate all whitespace checks to be the same, too.

https://github.com/Graylog2/graylog2-server/blob/0febd8f300578a238fbf1a8e8a89d2f3f2f46628/graylog2-server/src/main/java/org/graylog2/inputs/extractors/JsonExtractor.java#L129

https://docs.oracle.com/en/java/javase/17/docs/api/java.base/java/lang/Character.html#isWhitespace(char)

* Graylog Version: 6.2-SNAPSHOT