pkg/search: fix null "files" in file search response

The "files" field of a file search response was always non-null (empty JSON array in case of error or no result).
This behaviour was broken by df7cc0b453, so this CL is restoring it.

Fixes #619

Change-Id: I8e4762d1f5a2dba4f7cf06d8ca164392c4bf6d6a

pkg/search/handler.go

@@ -661,6 +661,11 @@ func (sh *Handler) serveFiles(rw http.ResponseWriter, req *http.Request) {
 		return
 	}
 
+	// the ui code expects an object
+	if files == nil {
+		files = []blob.Ref{}
+	}
+
 	ret.Files = files
 	return
 }

pkg/types/camtypes/search.go

@@ -250,5 +250,5 @@ type SearchErrorResponse struct {
 type FileSearchResponse struct {
 	SearchErrorResponse
 
-	Files []blob.Ref `json:"files,omitempty"` // Refs of the result files.
+	Files []blob.Ref `json:"files"` // Refs of the result files. Never nil.
 }