Allow for NIL value of dialect in ebib-utils.el

Specifically, the following functions are affected:

- ebib-list-fields
- ebib-list-undefined-fields
- ebib-list-entry-types
- ebib-list-fields-uniquely

These all depend on the BibTeX dialect. The calling functions get this
from the relevant database, but there the value of the dialect may be
NIL. In that case, the default value of `bibtex-dialect' should be used.