Add feature to filter fields with/without null value

[AbegaM]

Fixes #107

PR Sponsored by @IanMayo

Modifications

• Modified the comparison operator list with null and notNull operators
• Modified the SQL query to fetch null items

[IanMayo]

• [x] docs need to be updated too, please.
• [x] and maybe a unit test?

[AbegaM]

@IanMayo I will add a unit test in there and will modify the docs too

[IanMayo]

Hello @thevahidal - this is the last feature we're waiting for - so I'd appreciate that new release if/when you merge this.

Thanks!

[thevahidal]

Hey @IanMayo, I was waiting for your change requests about docs and tests, but Im ok to release if you're ok.

[AbegaM]

Hello @thevahidal I have added the unit tests and the docs are also modified