Do not set layout for no-op paragraph break

When breaking paragraph in an empty top-level paragraph, nothing
happens on screen but yet there is an undo step because the layout is
reset to what it already was.

Avoid this case.

Fixes bug #10089.

src/Text.cpp

@@ -743,6 +743,7 @@ void Text::breakParagraph(Cursor & cur, bool inverse_logic)
 		else {
 			docstring const & lay = cur.paragraph().usePlainLayout()
 			    ? tclass.plainLayoutName() : tclass.defaultLayoutName();
+			if (lay != layout.name())
 				setLayout(cur, lay);
 		}
 		return;