Allow enum value to be backed by an empty string

[martin-helmich]

Q	A
Documentation	no
Bugfix	yes
BC Break	no
New Feature	no
RFC	no
QA	no

Description

Currently, the EnumGenerator class enforces (in its Psalm typings) that a case of a backed enumeration is backed by int|non-empty-string. However, the non-empty-string constraint is unnecessarily strict, since (AFAIK)
an empty string is an entirely valid value (not name!) for a backed enum case.

This change relaxes the Psalm type hints a bit in order to allow this:

$enum = EnumGenerator::withConfig([
  "name"        => "Foo",
  "backedCases" => [
    "type"  => "string",
    "cases" => [
      "empty" => "",
      "foo"   => "foo",
      "bar"   => "bar",
    ],
  ]
]);

[Ocramius]

@martin-helmich CI shows that more adjustments are needed

[martin-helmich]

Yep, already noticed. Converting back to draft, for now. Although I suspect that (some) of the failing CI jobs may be unrelated to this change (this one, for example).

[martin-helmich]

Ready for review, again. The Psalm CI job now runs successfully; some PHPUnit jobs are failing for reasons I believe to be unrelated to this change.

[Ocramius]

Thanks @martin-helmich!