Improve error messaging with empty sourcetype, source

[Abigael-JT]

Description

%dpl
sourcetype=

%dpl
source=

Returns:
java.lang.IllegalStateException: Parser failure on line 2, column 11 due to no viable alternative at input 'sourcetype='
Please check that the query is written correctly. Otherwise, please report this error and include the query used and this error.

I think an empty index should also give an error message, currently it returns no results

Related ticket

• Improve error messaging with empty host #82