KLS: Don't discard an unresolved empty location during remapping.

This is a follow-up to #2944. In addition to not throwing away empty spans from
the service, also ensure that when those spans are not discarded when they
don't get moved by remapping.

kythe/go/languageserver/languageserver.go

@@ -307,12 +307,9 @@ func (ls *Server) TextDocumentDefinition(params lsp.TextDocumentPositionParams)
 				return []lsp.Location{loc}, nil
 			}
 
-			// There definition range doesn't exist anymore
-			return []lsp.Location{}, nil
+			// Their definition range doesn't exist anymore, or hasn't moved.
+			// Fall through and return the original location Kythe reported.
 		}
-
-		log.Printf("Unable to map definition to local location")
-		// We don't how to map it so we just return the location from Kythe
 		return []lsp.Location{*l}, nil
 	}