InlineStyle::getValue() skipped a declaration when empty($value) was true,
which also matches 0, 0.0 and '0'. Those are ordinary CSS values (opacity: 0,
z-index: 0, margin: 0, flex-grow: 0), so they were silently dropped, and a
style map containing only such declarations omitted the attribute entirely.
The sibling SeparatedTokenList::getValue() already uses an explicit
null/false test, so class token lists keep a 0 while style declarations did
not. The numeric-key branch of InlineStyle itself never consulted empty(),
so {style: ['opacity: 0']} printed while {style: {opacity: 0}} did not.