[css-ruby] size and position empty rbc/rtc (w3c#4941)

Closes w3c#4631

Author: frivoal

css-ruby-1/Overview.bs

@@ -649,6 +649,9 @@ Ruby Layout</h2>
 	exactly as if its [=ruby bases=] were a regular sequence of inline boxes.
 	Each [=ruby base container=] is sized and positioned
 	to contain exactly all of its [=ruby bases=]’ margin boxes.
+	If a [=ruby base container=] has no [=in-flow=] content,
+	it is sized and positioned
+	as if it contained a single empty [=ruby base=].
 
 	[=Ruby annotations=] associated with the [=base level=]
 	are then positioned with respect to their [=ruby base boxes=]
@@ -658,6 +661,9 @@ Ruby Layout</h2>
 	participating in the same inline formatting context.
 	Each [=ruby annotation container=] is sized and positioned
 	to contain exactly all of its [=ruby annotations=]’ margin boxes.
+	If a [=ruby annotation container=] has no [=in-flow=] content,
+	it is sized and positioned
+	as if it contained a single empty [=ruby annotation=].
 
 	[=Ruby annotation containers=] are stacked outward
 	over or under their corresponding [=ruby base container=],