Add isdefault + fix fqdn

Closes #299
Fixes #306

Author: Dean Karn

baked_in.go

@@ -1494,6 +1494,10 @@ func isHostname(fl FieldLevel) bool {
 func isFQDN(fl FieldLevel) bool {
 	val := fl.Field().String()
 
+	if val == "" {
+		return false
+	}
+
 	if val[len(val)-1] == '.' {
 		val = val[0 : len(val)-1]
 	}

validator_test.go

@@ -7189,6 +7189,7 @@ func TestFQDNValidation(t *testing.T) {
 		{"2001:cdba:0000:0000:0000:0000:3257:9652", false},
 		{"2001:cdba:0:0:0:0:3257:9652", false},
 		{"2001:cdba::3257:9652", false},
+		{"", false},
 	}
 
 	validate := New()