Unnecessary warning of missing input when adding text with -A

[niklas88]

When using the -A flag to add text data to an existing index, the IndexBuilderMain warns of a missing Knowledge Base input.

For example using command

 IndexBuilderMain -i /index/wikidata-2019-03-06 -A -w /input/wordsfile-2018-11-28-wikidata.tsv -d /input/docsfile-2018-11-28.tsv

results in the following warning

Tue Apr 16 10:33:59.189 - INFO:  No input knowledge-base-file specified. Parsing from stdin
Tue Apr 16 10:33:59.189 - WARN:   Could not deduce the type of the input knowledge-base-file by its extension. Assuming the input to be turtle. Please specify --file-format (-F)
Tue Apr 16 10:33:59.189 - WARN:  In case this is not correct

It looks like everything works anyway but the warning may confuse users. The fix would be to skip this warning when the -A flag is given