empty: add Equal empty string

[ccoVeille]

This pattern is common usef

assert.Equal(t, "", whatever)
assert.Equal(t, ``, whatever)
assert.Equal(t, "", string(whatever))
assert.Equal(t, ``, string(whatever))
assert.Empty(t, string(whatever))

And it could be replaced

assert.Empty(t, whatever)

Examples: https://github.com/search?q=language%3Ago+%22assert.Equal%28t%2C+%5C%22%5C%22%22&type=code https://github.com/search?q=language%3Ago+%22assert.Equal%28t%2C+%5C%22%5C%22%2C+string%22&type=code https://github.com/search?q=language%3Ago+%22assert.Empty%28t%2C+string%28%22&type=code

https://github.com/search?q=%22assert.Equal%28t%2C+%60%60%2C%22&type=code

https://github.com/search?q=%22assert.Equal%28t%2C+%60%60%2C+string%28%22&type=code