SA9003: disable by default

Flagging empty bodies in 'if' or 'else' blocks does sometimes find
missing code. However, it also prevents the useful idiom of documenting
why a branch isn't doing anything, as in

	if x {
		doSomething()
	} else {
		// If x isn't set, then so and so can't be the case and there's
		// nothing to do here.
	}

Over the years I've come to the conclusion that preventing these
comments is more harmful than the presence of accidentally empty
branches.

Updates: gh-321

staticcheck/doc.go

@@ -1142,10 +1142,11 @@ instead.`,
 	},
 
 	"SA9003": {
-		Title:    `Empty body in an if or else branch`,
-		Since:    "2017.1",
-		Severity: lint.SeverityWarning,
-		MergeIf:  lint.MergeIfAny,
+		Title:      `Empty body in an if or else branch`,
+		Since:      "2017.1",
+		NonDefault: true,
+		Severity:   lint.SeverityWarning,
+		MergeIf:    lint.MergeIfAny,
 	},
 
 	"SA9004": {