Clarify no preceding sibling case on ruby text content.

[spoeschel]

The ruby nesting constraints contain the following constraint:

"If the computed value of tts:ruby is text, then the computed value of tts:ruby of its preceding sibling is base, text, or delimiter"

This however will not work when complex ruby is used, as the first child of a ruby text container is ruby text content, but has no preceding sibling. See e.g. "Example Fragment – Double-sided Ruby",

So in line with the respective constraint related to ruby base content, the mentioned constraint should be changed to:

"If the computed value of tts:ruby is base, then its preceding sibling is either null (i.e., no preceding sibling) or the computed value of tts:ruby of its preceding sibling is base, text, or delimiter."

[skynavga]

If there is no preceding sibling, then the constraint doesn't apply. There is no such thing as a null preceding sibling.

[spoeschel]

If there is no preceding sibling, then the constraint doesn't apply

In my opinion the wording "of its preceding sibling" implies that such a sibling is actually present.

There is no such thing as a null preceding sibling.

Such wording is currently part of the TTML2 spec; see the second constraint that applies to tts:ruby when set to base:

"if the computed value of tts:ruby is base, then its preceding sibling is either null (i.e., no preceding sibling) or the computed value of tts:ruby of its preceding sibling is"

So this inconsistency has to be fixed somehow.