Show error if there's no secret to encode (#1580)

**Description of the change**
Kubeseal will throw an error when trying to seal no secrets. This will
happen when the file is empty or invalid, and the error suggests the
user to check the file encoding as this seems to be a common issue
especially in some versions of Windows #1410 #1560

**Benefits**
Give feedback to the user to help debugging a common issue

**Possible drawbacks**
Kubeseal will now throw an error where it previously just silently
succeeded with no output

**Applicable issues**
- fixes #1410
- fixes #1560

---------

Signed-off-by: Alejandro Moreno <[email protected]>

integration/kubeseal_test.go

@@ -189,6 +189,21 @@ var _ = Describe("kubeseal", func() {
 	})
 })
 
+var _ = Describe("kubeseal (with invalid input)", func() {
+	var input io.Reader
+	var output *bytes.Buffer
+	var args []string
+
+	BeforeEach(func() {
+		output = &bytes.Buffer{}
+	})
+
+	It("should throw an error", func() {
+		err := runKubeseal(args, input, output)
+		Expect(err).To(HaveOccurred())
+	})
+})
+
 var _ = Describe("kubeseal --fetch-cert", func() {
 	var c corev1.CoreV1Interface
 	var input io.Reader

pkg/kubeseal/kubeseal.go

@@ -189,6 +189,7 @@ func readSecrets(r io.Reader) ([]*v1.Secret, error) {
 			return nil, err
 		}
 	}
+
 	return secrets, nil
 }
 
@@ -226,6 +227,10 @@ func Seal(clientConfig ClientConfig, outputFormat string, in io.Reader, out io.W
 		return err
 	}
 
+	if len(secrets) == 0 {
+		return fmt.Errorf("no secrets found. Ensure the input is valid and UTF-8 encoded")
+	}
+
 	for _, secret := range secrets {
 		if len(secret.Data) == 0 && len(secret.StringData) == 0 && !allowEmptyData {
 			return fmt.Errorf("secret.data is empty in input Secret, assuming this is an error and aborting. To work with empty data, --allow-empty-data can be used")