Add a check if FieldMappers exists

[SergVro]

Closes #12572: Add a simple check if FieldMappers is not null

[rjernst]

@SergVro Can you add a test? You can see how it is tested on master here:
https://github.com/elastic/elasticsearch/blob/master/core/src/test/java/org/elasticsearch/search/query/ExistsMissingIT.java#L49

I think the same change should be in the MissingFilterParser as well?

[SergVro]

@rjernst I have added the same fix in the MissingFilterParser and tests for both filters

[nik9000]

Note: this fix should apply to 1.6 and 1.7.

[rjernst]

@SergVro Thanks, I merged this to 1.7. There won't be a 1.6.3 so I have not merged to there.