Make zipsync list file output empty if there are no files

The make javac rule uses [ -s srcjar-list ] to decide whether or
not to run javac.  zipsync was putting a trailing "\n" in the
list file, so the file was never empty.  There was one case of a
package that contained no source files (so java-source-list was
empty) and had a res directory with an xml file that declared no
resources (so aapt.jar was created but empty).  The 1-byte
srcjar-list file caused javac to run and error with:
javac: no source files

Test: m checkbuild
Change-Id: I26b394b66bf81e4f5abbd27e4dc06bee1d9420a8

cmd/zipsync/zipsync.go

@@ -118,7 +118,10 @@ func main() {
 	}
 
 	if *outputFile != "" {
-		data := strings.Join(files, "\n") + "\n"
+		data := strings.Join(files, "\n")
+		if len(files) > 0 {
+			data += "\n"
+		}
 		must(ioutil.WriteFile(*outputFile, []byte(data), 0666))
 	}
 }