Fix crash on a union type including null

[LeSuisse]

This change prevents the generation of type hints with 2 "null".

For example, "string|array|null" should generate the code "string|array|null" instead of "string|array|null|null" to avoid a PHP fatal error "Duplicate type null is redundant".

Closes #1105.

[GrahamCampbell]

Does that test pass without the code change? When I originally wrote this code string|null would not show up as a union type, and PHP would pretend it was ?string. I wonder if the bug you were experiencing is a bug with union types in core code, and not userland functions?

[LeSuisse]

No it does not, see https://travis-ci.com/github/LeSuisse/mockery/jobs/452045803 which correspond to LeSuisse@d328ed6 (the test without the code change).
I detected the issue by mocking a core class but it seems userland is also affected.

[LeSuisse]

The CI failures with DEPS=lowest on PHP 7.3 and PHP 7.4 seems to be unrelated to this change (I did not confirm it but it looks like a side effect of the release of Xdebug 3.0).

[GrahamCampbell]

NB This PR should target the 1.3 branch. PHPUnit issues can be fixed by replacing ^5.7.10|^6.5|^7.5|^8.5|^9.3 with the same thing, only with each major version min changed to very latest in that series.

[LeSuisse]

The PR now targets the 1.3 branch.

Not exactly sure to understand how to fix the PHPUnit/Xdebug 3.0 issues on it since we need PHPUnit ^5 .7 to run the tests on PHP 5.6 and this version also covers PHP ^7.0.

[GrahamCampbell]

Hmm, mockery should probably just drop it's prefer-lowest tests. They don't add any value, since Mockery has basically no dependencies.

[davedevelopment]

This looks good to me and I'm happy to drop the prefer-lowest tests, to be honest that all confuses me anyway.

[LeSuisse]

I opened #1107 to drop the prefer-lowest tests. I will rebase this PR once the other is merged :) .

[davedevelopment]

Sorry for the delay, just tagged 1.4.3 and 1.3.4