InputService.find() throws exception when record is not found.

This prevents an NPE when find() is called for non-existing input
ids.

Fixes #684
Source input of message might be null when input was deleted before.
FQCN not needed here.
Replacing wildcard imports with explicit ones.
Merge pull request #686 from Graylog2/issue684

InputService.find() throws exception when record is not found.