JsonPathSegmentVariableExtractor and XPathSegmentVariableExtractor fail to resolve "null" values

**Citrus Version**
4.8

**Expected behavior**
The extractors should successfully extract an existing null value from the source

Also, the error message could be improved. It is quite hard to understand what is happening, when the extractor fails.

**Actual behavior**
The extractors fail with "unresolvable" message.

**Test case sample**
> Please, share the test case (as small as possible) which shows the issue