resource/aws_instance: Add check for empty credit_specification block

Fixes: #6532

When given an empty credit_specification block (i.e cpu_credits is missing) the Terraform provider
will crash upon trying to type assert on the empty (nil) block. This check introduces a guard clause around the type assertion
to protect around the empty (nil) block. A warning is logged to indicated that a default value will be used in place of the missing
value.

Acceptance tests after changes
```
--- PASS: TestAccAWSInstance_creditSpecification_unknownCpuCredits_t2
--- PASS: TestAccAWSInstance_creditSpecification_unknownCpuCredits_t3
```

aws/resource_aws_instance.go

@@ -1764,9 +1764,12 @@ func buildAwsInstanceOpts(
 	if v, ok := d.GetOk("credit_specification"); ok {
 		// Only T2 and T3 are burstable performance instance types and supports Unlimited
 		if strings.HasPrefix(instanceType, "t2") || strings.HasPrefix(instanceType, "t3") {
-			cs := v.([]interface{})[0].(map[string]interface{})
-			opts.CreditSpecification = &ec2.CreditSpecificationRequest{
-				CpuCredits: aws.String(cs["cpu_credits"].(string)),
+			if cs, ok := v.([]interface{})[0].(map[string]interface{}); ok {
+				opts.CreditSpecification = &ec2.CreditSpecificationRequest{
+					CpuCredits: aws.String(cs["cpu_credits"].(string)),
+				}
+			} else {
+				log.Print("[WARN] credit_specification is defined but the value of cpu_credits is missing, default value will be used.")
 			}
 		} else {
 			log.Print("[WARN] credit_specification is defined but instance type is not T2/T3. Ignoring...")

aws/resource_aws_instance_test.go

@@ -1693,6 +1693,50 @@ func TestAccAWSInstance_creditSpecification_unlimitedCpuCredits(t *testing.T) {
 	})
 }
 
+func TestAccAWSInstance_creditSpecification_unknownCpuCredits_t2(t *testing.T) {
+	var instance ec2.Instance
+	rInt := acctest.RandInt()
+	resName := "aws_instance.foo"
+
+	resource.ParallelTest(t, resource.TestCase{
+		PreCheck:     func() { testAccPreCheck(t) },
+		Providers:    testAccProviders,
+		CheckDestroy: testAccCheckInstanceDestroy,
+		Steps: []resource.TestStep{
+			{
+				Config: testAccInstanceConfig_creditSpecification_unknownCpuCredits(rInt, "t2.micro"),
+				Check: resource.ComposeTestCheckFunc(
+					testAccCheckInstanceExists(resName, &instance),
+					resource.TestCheckResourceAttr(resName, "credit_specification.#", "1"),
+					resource.TestCheckResourceAttr(resName, "credit_specification.0.cpu_credits", "standard"),
+				),
+			},
+		},
+	})
+}
+
+func TestAccAWSInstance_creditSpecification_unknownCpuCredits_t3(t *testing.T) {
+	var instance ec2.Instance
+	rInt := acctest.RandInt()
+	resName := "aws_instance.foo"
+
+	resource.ParallelTest(t, resource.TestCase{
+		PreCheck:     func() { testAccPreCheck(t) },
+		Providers:    testAccProviders,
+		CheckDestroy: testAccCheckInstanceDestroy,
+		Steps: []resource.TestStep{
+			{
+				Config: testAccInstanceConfig_creditSpecification_unknownCpuCredits(rInt, "t3.micro"),
+				Check: resource.ComposeTestCheckFunc(
+					testAccCheckInstanceExists(resName, &instance),
+					resource.TestCheckResourceAttr(resName, "credit_specification.#", "1"),
+					resource.TestCheckResourceAttr(resName, "credit_specification.0.cpu_credits", "unlimited"),
+				),
+			},
+		},
+	})
+}
+
 func TestAccAWSInstance_creditSpecification_updateCpuCredits(t *testing.T) {
 	var first, second, third ec2.Instance
 	resName := "aws_instance.foo"
@@ -3827,6 +3871,7 @@ resource "aws_instance" "foo" {
   ami           = "ami-51537029"               # us-west-2
   instance_type = "t3.micro"
   subnet_id     = "${aws_subnet.my_subnet.id}"
+
 }
 `, rInt)
 }
@@ -3971,6 +4016,33 @@ resource "aws_instance" "foo" {
 `, rInt)
 }
 
+func testAccInstanceConfig_creditSpecification_unknownCpuCredits(rInt int, instanceType string) string {
+	return fmt.Sprintf(`
+resource "aws_vpc" "my_vpc" {
+  cidr_block = "172.16.0.0/16"
+
+  tags = {
+    Name = "tf-acctest-%d"
+  }
+}
+
+resource "aws_subnet" "my_subnet" {
+  vpc_id            = "${aws_vpc.my_vpc.id}"
+  cidr_block        = "172.16.20.0/24"
+  availability_zone = "us-west-2a"
+}
+
+resource "aws_instance" "foo" {
+  ami           = "ami-51537029"               # us-west-2
+  instance_type = %q
+  subnet_id     = "${aws_subnet.my_subnet.id}"
+
+  credit_specification {
+  }
+}
+`, rInt, instanceType)
+}
+
 func testAccInstanceConfig_UserData_Base(rInt int) string {
 	return fmt.Sprintf(`
 data "aws_ami" "amzn-ami-minimal-hvm-ebs" {