ExternalFileField/FileFloatSource throws NPE if keyField is Points based

FileFloatSource (and by extension ExternalFileField) currently throws an NPE if the keyField is Points based.

This seems to be due to hardcoded assumptions about MultiFields.getTerms(...) returning a non-null value for the keyField (which I suspect could also lead to an NPE in situations where the index is completley empty? ... not sure)

In general, the error handling in FileFloatSource should be improved.

If there is no straight forward way to make FileFloatSource work with Point fields, then ExternalFileField should throw a clear error on init if the true==keyFieldType.isPointField()