fix(database): allow null values for equalTo, etc. - #809

Checklist

• Issue number for this PR: Queries with null don't work as expected #704 and fix(database): allow null values for equalTo, etc. #715
• Docs included?: no
• Test units included?: yes
• e2e tests included?: no
• In a clean directory, npm install, npm run build, and npm test run successfully? yes

Description

The previous PR for #704 did not include tests and did not completely fix the issue, as null values were swallowed with the query observable. This PR addresses that and includes tests.

Also, this PR ensures that the query observable emits undefined if values are not specified for equalTo, startAt or endAt - as null has a specific meaning.