[0.7-M5] Calling `setParagraphStyle` on an empty Paragraph [p.length() == 0] throws Exception

[neilccbrown]

With version 0.7M5, if I call setParagraphStyle, I get an exception saying that it's invalid to have a (rich text) change which doesn't add or remove text. This seems like a bug given that setParagraphStyle does exactly that, and in general it should be valid to have a rich text change which only changes styles. Stack trace:

java.lang.IllegalStateException: Cannot get the type of a change that neither inserts nor deletes anything.
	at org.fxmisc.richtext.model.TextChange.<init>(TextChange.java:31)
	at org.fxmisc.richtext.model.RichTextChange.<init>(RichTextChange.java:6)
	at org.fxmisc.richtext.model.ReadOnlyStyledDocument.lambda$null$7(ReadOnlyStyledDocument.java:274)
	at org.reactfx.util.Tuple2.map(Tuple2.java:31)
	at org.fxmisc.richtext.model.ReadOnlyStyledDocument.lambda$replace$8(ReadOnlyStyledDocument.java:271)
	at org.reactfx.util.Tuple2.map(Tuple2.java:31)
	at org.fxmisc.richtext.model.ReadOnlyStyledDocument.replace(ReadOnlyStyledDocument.java:270)
	at org.fxmisc.richtext.model.ReadOnlyStyledDocument.replaceParagraph(ReadOnlyStyledDocument.java:285)
	at org.fxmisc.richtext.model.GenericEditableStyledDocumentBase.setParagraphStyle(GenericEditableStyledDocumentBase.java:179)
	at org.fxmisc.richtext.model.SimpleEditableStyledDocument.setParagraphStyle(SimpleEditableStyledDocument.java:7)

Another regression introduced in #458?

[JordanMartinez]

:-/ Yup, I forgot about the possibility of a style change

This is another issue that would have been found had I tested the code using the rich text demo before merging it into master. However, that just shows that we need to have more automated tests that test the basic functionality of these methods.

[JordanMartinez]

This regression only appears if the paragraph does not have any text on it whatsoever.