Fix BiMap and InverseMap `get` type to allow `undefined` return

[rubenferreira97]

BiMap and InverseMap were wrongly typed.

Example:

import { BiMap } from 'mnemonist';

const a = new BiMap<string, string>();
a.set('a', 'b');
console.log(a.get('b')); // undefined
console.log(a.inverse.get('a')); // undefined

[Yomguithereal]

Thanks @rubenferreira97 merging right now.

[Yomguithereal]

Now published in v0.39.4