[TypeScript ]Add missing types for havingNull and havingNotNull

[ducher]

The documentation has the havingNull and havingNotNull methods but they are missing from the TS definition so I added them along with a simple type test.

Tell me if my PR is missing anything

[OlivierCavadenti]

thanks !

[coveralls]

Coverage: 92.392%. Remained the same when pulling 313ec19 on ducher:fix/havingNull-typings into 42b9198 on knex:master.

[ducher]

The CI is throwing this error, any idea what can cause this ? I don't see any JSDoc tag in my changes

The 'no-redundant-jsdoc' rule threw an error in '/home/runner/work/knex/knex/types/index.d.ts':
Error: Unexpected tag kind: JSDocDeprecatedTag
    at checkTag (/home/runner/work/knex/knex/node_modules/tslint/lib/rules/noRedundantJsdocRule.js:105:23)
    at cb (/home/runner/work/knex/knex/node_modules/tslint/lib/rules/noRedundantJsdocRule.js:62:25)
    at visitNodes (/home/runner/work/knex/knex/node_modules/typescript/lib/typescript.js:31095:30)
    at Object.forEachChild (/home/runner/work/knex/knex/node_modules/typescript/lib/typescript.js:31502:21)
    at cb (/home/runner/work/knex/knex/node_modules/tslint/lib/rules/noRedundantJsdocRule.js:67:19)
    at visitNodes (/home/runner/work/knex/knex/node_modules/typescript/lib/typescript.js:31095:30)
    at Object.forEachChild (/home/runner/work/knex/knex/node_modules/typescript/lib/typescript.js:31421:24)
    at cb (/home/runner/work/knex/knex/node_modules/tslint/lib/rules/noRedundantJsdocRule.js:67:19)
    at visitNode (/home/runner/work/knex/knex/node_modules/typescript/lib/typescript.js:31086:24)
    at Object.forEachChild (/home/runner/work/knex/knex/node_modules/typescript/lib/typescript.js:31521:21)

Only way to make it work for me is to remove all the comments with @deprecated in the index.d.ts file but they were already there, did the tslint rules change recently ?

EDIT: Sorry I just saw that I wrote the tests in the wrong file!