Fix NPE when searching after deleting docs

[alexklibisz]

Based on #158

So far it seems that the solution is:

1. Explicitly handle the possibility of null returned when calling leafReader.terms(field).
2. Use leafReader.numDocs instead of indexReader.numDocs to determine the total number of possible docs.

Added a test for this scenario: index some docs, run a search, delete one doc, run a search, put the doc back, run a search. This passes locally.