Fix « empty » checkbox value

[dugwood]

Hello!

(new PR to use the 5.0.x branch)

• Type: bug fix
• Link to issue: [BUG]: Attribute 'checked' in Check element #15959

In raising this pull request, I confirm the following:

• I have read and understood the Contributing Guidelines
• I have checked that another pull request for this purpose does not exist
• I wrote some tests for this PR
• I have updated the relevant CHANGELOG
• I have created a PR for the documentation about this change

Small description of change:
Setting checked attribute to an « empty » value doesn't check the checkbox/radio input. « empty » is understand as the return of « empty() » PHP function.

For example 0 and the empty string are considered empty, but <input type="checkbox" name="test" value="0" checked="checked"/> is a valid option that Phalcon 5 doesn't allow.

Thanks

[niden]

Thank you @dugwood