Check Type::isAssignable() implementations with regard to nullability

[sebastianbergmann]

@stof wrote:

2 types allowing null are not automatically assignable to each other. Null is assignable to a type allowing null. And a type allowing null is not assignable to a type not allowing null. But ?string is not assignable to ?int

[stof]

Btw, infection pushing you to perform such changes and the testsuite accepting it seems to indicate that the test coverable of isAssignable is incomplete.

[sebastianbergmann]

The changes that were prompted by Infection have been reverted. The isAssignable() implementation are now as they were. They likely still need more tests, but I'm closing this ticket now.