flag: simplify switch-case in isZeroValue

Simplifies the switch statement in the isZeroValue function by merging
the case branches.

Change-Id: I5b27939b62808dadac0cef632567b17e0e2b9a1d
Reviewed-on: https://go-review.googlesource.com/71390
Run-TryBot: Gabriel Aszalos <gabriel.aszalos@gmail.com>
TryBot-Result: Gobot Gobot <gobot@golang.org>
Reviewed-by: Ian Lance Taylor <iant@golang.org>

src/flag/flag.go

@@ -400,11 +400,7 @@ func isZeroValue(flag *Flag, value string) bool {
 	}
 
 	switch value {
-	case "false":
-		return true
-	case "":
-		return true
-	case "0":
+	case "false", "", "0":
 		return true
 	}
 	return false