test: ErrorIs should support having both err and target be nil

[SpikesDivZero]

Go's built-in errors.Is returns true when both target and err are nil, but this library marks the test as a failure.

I came across this while writing a table-based test, in which I had to work around this by writing:

test.True(t, errors.Is(err, tt.wantErrIs))
// -- OR --
if tt.wantErrIs == nil {
	test.Nil(t, err)
} else {
	test.ErrorIs(t, err, tt.wantErrIs)
}

After this change is applied, the test code can be reduced to a clear statement instead.

[shoenig]

LGTM; thanks @SpikesDivZero!