[5.2] Accept != and <> as operators while value is null

[themsaid]

As noted here #13314

Currently:

->where('done_at','=',null); // converted to whereNull
->where('done_at', '!=', null) // throws exception

This PR accepts != and <> as operators when the value given to where() is null, allowing the conversion of where with these operators to have the same effect as whereNotNull.

[srmklive]

@themsaid There are already whereNull & whereNotNull functions available in query builder, then why is there a reason for changes for the where function to encompass the behavior of whereNull?

[themsaid]

@srmklive where() accepts null as a value already, this PR just adds support for the != and <> operators.

[srmklive]

@themsaid I am currently using both the != && <> operator in the where function in one of my apps. Haven't encountered any error you mentioned yet.

[themsaid]

@srmklive if the value given is null, using these operators will throw an exception.