Fix issue 61614

[msteed]

What does this PR do?

Makes state.show_state_usage not fail when a saltenv contains states but is not referenced in any topfile.

What issues does this PR fix or reference?

Fixes: #61614

Previous Behavior

Create a saltenv with a state and an empty topfile. Then salt \* state.show_state_usage results in:

self = <salt.state.HighState object at 0x7f1b306c7550>

    def compile_state_usage(self):
        """
        Return all used and unused states for the minion based on the top match data
        """
        err = []
        top = self.get_top()
        err += self.verify_tops(top)

        if err:
            return err

        matches = self.top_matches(top)
        state_usage = {}

        for saltenv, states in self.avail.items():
            env_usage = {
                "used": [],
                "unused": [],
                "count_all": 0,
                "count_used": 0,
                "count_unused": 0,
            }

            env_matches = matches.get(saltenv)

            for state in states:
                env_usage["count_all"] += 1
>               if state in env_matches:
E               TypeError: argument of type 'NoneType' is not iterable

New Behavior

No failure

Merge requirements satisfied?

[NOTICE] Bug fixes or features added to Salt require tests.

• Docs
• Changelog - https://docs.saltproject.io/en/master/topics/development/changelog.html
• Tests written/updated

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Yes

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