Fix bug and add test for empty text handling in styled bidirectional processing

[Omkarthipparthi]

Fix return format for processStyledBidirectionalText with empty input

The processStyledBidirectionalText function was returning an incorrect format when given empty text. It returned [{text, styleIndices}] (an object wrapped in an array) instead of [[text, styleIndices]] (a tuple/array wrapped in an array).

This caused issues in maplibre-gl-js when both name and ref fields were empty (see maplibre/maplibre-gl-js#6444).

Changes:

• Fixed line 162 in src/index.js to return [[text, styleIndices]]
• Added test case to prevent regression

The fix ensures consistency with the function's documented return type: Array<[string, Array<number>]>

[HarelM]

@stepankuzmin @ThibaudLopez can you guys take a look at this PR and hopefully merge and release a new version?

Thanks @Omkarthipparthi!