[FIRRTL] 1.60 newly warns about empty (top) module twice, should be zero or at most once

This is new as of 1.60 and does not occur with 1.59.

Previously: #6065 .

Input:
```firrtl
circuit Empty:
module Empty:
```

Latest firtool new prints two instead of zero warnings:

```
empty.fir:2:3: warning: module `Empty` is empty but cannot be removed because the module is public
module Empty:
^
empty.fir:2:3: warning: module `Empty` is empty but cannot be removed because the module is public
module Empty:
^
```

This certainly shouldn't be twice and probably shouldn't be done for any public module.

Reproduce the warning with firtool 1.60 using the provided empty.fir input, then compare the output with version 1.59. Trace the handling of empty public modules and add or update coverage so the warning is emitted zero or at most once, then rerun the reproduction.

Written by the indexing model from the issue text.