empty: equal empty slice support

[NathanBaulch]

I'm completely new to Go code analysis but thought I'd give this a try.

I have a project where contributors often assert values are equal to empty slices. I'd like these to use assert.Empty for more meaningful failure messages and general consistency.

assert.Equal(t, []foo{}, actual) -> assert.Empty(actual)

There are a couple of complications to consider however. If the actual value is a nil slice then assert.Empty is more permissive since assert.Equal would have failed. This is not a distinction I personally care about, but I'm not sure how conservative this linter aims to be. See stretchr/testify#1141.

actual := ([]string)(nil)
assert.Equal(t, []string{}, actual)  // fail
assert.Empty(t, actual)              // success

Also, if the types don't match then that would also produce a false-success.

var actual any = []int{}
assert.Equal(t, []string{}, actual)  // fail
assert.Empty(t, actual)              // success

It looks easy enough to add a type check with something like types.Identical(pass.TypesInfo.TypeOf(a), pass.TypesInfo.TypeOf(b)) but the other equals based empty checks don't do this so I left it out. Perhaps they should?