Fix cover bug on last expressions with empty clauses

OTP-8188 introduced a fix for handling last expressions in
expressions like case, try and friends. However the fix did
not account that some of those expressions like receive may
have no clauses (only an after clause), leading to a function
clause error when cover compiling code with such expressions.
Merge branch 'josevalim/jv-cover-last-expr-maint/OTP-12328' into maint

* josevalim/jv-cover-last-expr-maint/OTP-12328:
  Fix cover bug on last expressions with empty clauses