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

[dtzSiFive]

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

Previously: #6065 .

Input:

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.