Fix panic for unknown and null values (#530)

Author: wata727

rules/aws_resource_missing_tags.go

@@ -379,9 +379,12 @@ func stringInSlice(a string, list []string) bool {
 // If _any_ key is unknown, the entire value is considered unknown, since we can't know if a required tag might be matched by the unknown key.
 // Values are entirely ignored and can be unknown.
 func getKeysForValue(value cty.Value) (keys []string, known bool) {
-	if !value.CanIterateElements() {
+	if !value.CanIterateElements() || !value.IsKnown() {
 		return nil, false
 	}
+	if value.IsNull() {
+		return keys, true
+	}
 
 	return keys, !value.ForEachElement(func(key, _ cty.Value) bool {
 		// If any key is unknown or sensitive, return early as any missing tag could be this unknown key.

rules/aws_resource_missing_tags_test.go

@@ -406,7 +406,7 @@ rule "aws_resource_missing_tags" {
 			Expected: helper.Issues{},
 		},
 		{
-			Name: "Unkown value maps should be silently ignored",
+			Name: "Unknown value maps should be silently ignored",
 			Content: `variable "aws_region" {
   default = "us-east-1"
   type = string