Add N.C. entry to ChordNames context.

input/regression/chordnames-nochord.ly

 \version "2.13.1"
 
 \header {
   texidoc = "Rests in music passed to ChordNames context display noChordSymbol.
 noChordSymbol is treated like a ChordName with respect to chordChanges.
 "
 }
 
 myChords = \chordmode {
   c1 r1 r1 \break
-  r1 g1 c1
+  r1 g1 c1 \break
 }
 
 \score {
   <<
-    \new ChordNames { \myChords }
-    \new FretBoards { \myChords }
-    \new Staff { \myChords }
+    \new ChordNames {·
+      \myChords·
+      \set chordChanges = ##t
+      \myChords
+    }
+    \new FretBoards {·
+      \myChords·
+      \myChords
+    }
+    \new Staff {·
+      \myChords·
+      \myChords
+    }
   >>
 }