PHPLIB-980: Make uri parameter for MongoDB\Client::__construct nullable

[alcaeus]

PHPLIB-980

I decided to only make the $uri parameter nullable. While we could widen the types of both options parameters (constructors are excluded from contravariance checks), I believe this is contra-productive. The signature of MongoDB\Manager historically allowed null, but it's preferable to not allow null and instead default to an empty array. This is something I'd like to change in version 2.0 of the extension.

This also becomes a non-issue starting with PHP 8, as users can use named parameters if they want to only pass a single option:

$client = new Client(driverOptions: ['foo' => 'bar']);

This makes using null as an easy default value easier.

For convenience and documentation purposes, I've also extracted the default URI into a public constant.

[jmikola]

The $uri docs on https://www.mongodb.com/docs/php-library/current/reference/method/MongoDBClient__construct/ already state:

Defaults to "mongodb://127.0.0.1:27017" if unspecified.

So there's no need to change anything else on this page. I also don't think there's any need to document the constant you introduced.