fix: prevent style={{}} from destroying computed styles in non-["style"] targets (#312)

When a component passes an empty style object (e.g., contentContainerStyle={{}})
to a non-["style"] target, mergeDefinedProps copies the empty object over the
computed className styles, destroying them. This is the same class of bug as
style={undefined} (fixed in PR #291) but for empty objects.

The ["style"] target path (Path A) already handles this correctly via
filterCssVariables({}) returning undefined. This fix extends mergeDefinedProps
with an isEmptyPlainObject check to skip empty objects on paths B and C,
matching Path A's behavior.

Common real-world trigger: components with default empty object parameters
({ contentContainerStyle = {} }) or explicit style={{}} props.