Fix issue 21199: call unaryFun!fun in Nullable.apply to support string parameter.

[FeepingCreature]

Fixes Nullable.apply!"a.foo" not working.

Mea culpa!

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Bugzilla references

Auto-close	Bugzilla	Severity	Description
✓	21199	trivial	Nullable.apply should take string expression like rest of Phobos

Testing this PR locally

If you don't have a local development environment setup, you can use Digger to test this PR:

dub run digger -- build "master + phobos#7606"