Handle nil and "" values equally

[jonatanklosko]

Phoenix.HTML.Form.options_for_select([x: "x", empty: ""], "")
#=> <option value="x">x</option>
#=> <option selected value="">empty</option>

# vs

Phoenix.HTML.Form.options_for_select([x: "x", empty: ""], nil)
#=> <option value="x">x</option>
#=> <option value="">empty</option>

This discrepancy comes from the fact that we use html_escape to normalize values, and it it's different for nil and "".