Fix html_attr dropping style declarations whose value is zero

[dylanpulver]

InlineStyle::getValue() skips a declaration when empty($value) is true. That also matches 0, 0.0 and '0', which are ordinary CSS values.

{{ html_attr({style: {opacity: 0}}) }}                    {# "" #}
{{ html_attr({style: {'flex-grow': 0, color: 'red'}}) }}  {# style="color: red;" #}
{{ html_attr({style: ['opacity: 0']}) }}                  {# style="opacity: 0;" #}
{{ html_attr({class: [0, 'a']}) }}                        {# class="0 a" #}

Same declaration, opposite result. The numeric-key branch never consults empty(), and the sibling SeparatedTokenList::getValue() already uses an explicit null !== $v && false !== $v test, so token lists keep a 0. When every declaration is dropped the attribute is omitted entirely, so {style: {opacity: 0}} renders nothing at all, and HtmlExtension::htmlAttrValue('style', ['opacity' => 0]) returns null.

Silently affects opacity, z-index, margin, padding, border, flex-grow and custom properties.

The guard now lists the values that carry no declaration. null, false, '' and true still skip, and [] is kept in that list so empty arrays behave exactly as before.

html_attr.rst documents the null/false/true omission rules and says nothing about zero, so no doc change is needed. HtmlAttrTest.php already has a case named "zero is not treated as falsy", but only for a plain attribute value.

3 tests added. Reverting the fix fails the first; a naive null/false-only guard fails the other two.

[fabpot]

Thank you @dylanpulver.