fix: RequiresEnvironmentVariable should only allow strings

[nikophil]

see #6074 (review)

some questions towards the behavior of the feature:

• should I only test $_ENV or maybe $_ENV ?? getenv()?
• when no value is provided, should I at least test the value of the env var is okish?
• not sure what @mvorisek meant here by "in separate process"?

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 94.59%. Comparing base (7296d39) to head (aa67817).
Report is 1 commits behind head on main.

Additional details and impacted files

@@            Coverage Diff            @@
##               main    #6079   +/-   ##
=========================================
  Coverage     94.59%   94.59%           
- Complexity     6632     6634    +2     
=========================================
  Files           709      709           
  Lines         20862    20863    +1     
=========================================
+ Hits          19734    19735    +1     
  Misses         1128     1128           

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[sebastianbergmann]

when no value is provided, should I at least test the value of the env var is okish?

You mean when RequiresEnvironmentVariable is used without an expected value and only the existence of an environment variable is required? In that: no, I do not think that we need to perform any check beyond whether or not an environment variable of the specified name exists.

[sebastianbergmann]

should I only test $_ENV or maybe $_ENV ?? getenv()?

I almost never work with environment variables, so bear with me please: is there a situation where an environment variable is not accessible through $_ENV but is accessible through getenv() or vice versa?

[nikophil]

I do not think that we need to perform any check beyond whether or not an environment variable of the specified name exists

Yes, that's what I've done.
The only check I've made is to ensure the env var is not an empty string, as suggested by @mvorisek

is there a situation where an environment variable is not accessible through $_ENV but is accessible through getenv() or vice versa?

I think when it is manually set, an env var can be added to only one or both: if only added to $_ENV, it is not accessible with getenv() and vice versa. symfony/dotenv does not even use putenv() by default