fix: reject a whitespace-only value in isBoolean()

[raphyabak]

Summary

Fixes #611

filter_var() with FILTER_VALIDATE_BOOLEAN trims its input before matching, so a value that is only spaces or tabs has nothing left to match and is treated as the valid boolean false — it is not a "failure" that FILTER_NULL_ON_FAILURE would catch:

var_dump(filter_var('   ', FILTER_VALIDATE_BOOLEAN, FILTER_NULL_ON_FAILURE)); // bool(false), not null

isBoolean() only guarded against the exact empty string ($value === ''), so a value that is empty only after trimming slipped past that guard and was then accepted as false by filter_var(). A genuinely empty value already failed correctly, since '' === '' is true — only whitespace-only values slipped through.

// FLAG="   " in the environment
$dotenv->required('FLAG')->isBoolean();   // passed, but should not
// FLAG= (empty) already correctly failed

Fix

Trim before the empty check, mirroring the guard notEmpty() already uses elsewhere in this same class, so a whitespace-only value is rejected the same way an empty one is.

As noted in the issue, this is a behaviour change — a whitespace-only value that currently passes validation would start failing — so it targets master rather than a 5.x patch release.

Test plan

• Added INVALID_WHITESPACE=" " to tests/fixtures/env/booleans.env and wired it into invalidBooleanValuesDataProvider(), so it's exercised by the existing testCanInvalidateNonBooleans / testCanInvalidateNonBooleansIfPresent tests alongside every other invalid-boolean case.
• Verified both new data-provider cases fail against the pre-fix code with the exact reported symptom (isBoolean() passing a whitespace-only value), and pass with the fix.
• Ran the full test suite (vendor/bin/phpunit): 282 tests, all passing (280 pre-existing + 2 new).
• vendor/bin/phpstan analyze (configured scope: src, level max): no errors.

[GrahamCampbell]

This is a major breaking change, which is why I opened an issue to track this, instead of implementing it. Please do not just point AI an issue without maintainer approval - if I wanted to do that, I could do that myself.