Normalize splatted merge arrays like explicit attributes

[OskarEichler]

Summary

Normalize arrays used in splatted mergeable attributes the same way as explicit attributes: flatten, convert elements to strings, remove empty values, then join. This also omits an attribute when the resulting value is empty.

Reproduction

values = ['a', nil, '', ['b', '']]
Slim::Template.new { 'p class=values' }.render(nil, values: values)
Slim::Template.new { 'p *attrs' }.render(nil, attrs: {class: values})

Before: direct output has class="a b", splatted output has class="a b ". An all-empty splat array produces class=" " rather than omitting the attribute. After: both paths match. Existing conversion of true/false to strings within merge arrays is unchanged.

Verification

• Unmodified core/plugin suites: 415 runs / 650 assertions passing on Ruby 3.2.11 and 4.0.6; Rails integration 15 runs / 36 assertions passing on each.
• 2,700 focused equivalence/input checks per Ruby across seeded nested/empty/nil/boolean/special-character values, three merge delimiters and class/id/custom attributes, including frozen inputs.
• Syntax passes and no new comparative Lint offenses. No test files changed.

Compatibility / limitations

No API removal or dependency upgrade. Output intentionally changes for splatted merge arrays containing empty/nil nested values; code relying on extra whitespace or empty pieces with custom delimiters should account for this. This follows the existing direct-attribute semantics. General data/aria boolean policy (#872/#974) is unchanged.