Update tsconfig to enable strict null checks

[fnimick]

There have been multiple issues (#481 , #483 ) caused by the inability to develop the library with strict null checks enabled. These issues do not appear in library development but do appear in client code running with strict checks enabled.

I personally attempted to add strict null check support as part of #482 but was not able to as I don't have the experience necessary with the library to correctly add null checks where necessary.

https://www.typescriptlang.org/tsconfig#strictNullChecks