backend chokes on ! in text-query

[nleanba]

GET /doc?text=!abc and GET /doc?text=! produce an Error 500

Orgin of problem:

Malformed sparql-query, because tridoc-backend logs FetchError: invalid json response body at http://fuseki:3030/3DOC/query reason: Unexpected token E in JSON at position 0

and fuseki logs:

... // Long Error stack
Caused by: org.apache.jena.query.text.TextIndexParseException: Text search parse error:
Cannot parse 'name:!': Encountered " <NOT> "! "" at line 1, column 5.