Fix parseDate return type not nullable

[goodoldneon]

Fix parseDate returning Date instead of Date | null in its types.

Setting "strictNullChecks": true in tsconfig.json will prevent regressions, but that requires some refactoring to satisfy. There are a handful of places that:

• Expect null but get a value that could be undefined
• Expect undefined but get a value that could be null
• Expect a non-nullish type but get a value that could be nullish

[coveralls]

coverage: 91.378%. remained the same
when pulling 3126f44 on goodoldneon:fix-parseDate-type
into 1f9f741 on wanasit:master.

[wanasit]

Thanks for the fix!