Porting note: unimplemented has_nonempty_instance linter

[grunweg]

This issue is tracking all porting notes referring to the has_nonempty_instance linter from mathlib3:
as this linter has not been ported to mathlib4 yet, the nolint entries from mathlib3 are commented. Porting notes look like

mathlib4/Mathlib/Algebra/Algebra/Hom.lean

Line 31 in b0aea61

-- @[nolint has_nonempty_instance] -- Porting note: This linter does not exist yet.

Steps to fix

• port the linter
• see which of the previous nolints are required: uncomment the necessary ones and delete the now-superfluous ones.
  Remove the porting note in either case.
• deal with any new errors in mathlib4
• land the linter and rejoice :-)

[grunweg]

Closing in favour of #5171, oops.