config3alphato3: Default domain to empty string (#5015)

Update convertConfig3AlphaTo3() to use an empty string as a domain name
when the read config has no domain information.

Check the existence of "domain" field in the config before attempting a
type assertion. This helps avoid panic:

```
panic: interface conversion: interface {} is nil, not string
```

Signed-off-by: Sunny <[email protected]>

Author: darkowlzz

internal/cmd/operator-sdk/alpha/config3alphato3/convert_config_3-alpha_to_3.go

@@ -66,7 +66,12 @@ func convertConfig3AlphaTo3(cfgBytes []byte) (_ []byte, err error) {
 	isMultigroup := hasMultigroup && multigroupObj.(bool)
 
 	if obj, hasRes := cfgObj["resources"]; hasRes {
-		domain := cfgObj["domain"].(string)
+		// Default to empty domain if no domain information is found.
+		domain := ""
+		if domainObj, ok := cfgObj["domain"]; ok {
+			domain = domainObj.(string)
+		}
+
 		resObjs := obj.([]interface{})
 		resources := make([]resource.Resource, len(resObjs))
 

internal/cmd/operator-sdk/alpha/config3alphato3/convert_config_3-alpha_to_3_test.go

@@ -36,6 +36,7 @@ var _ = Describe("ConvertConfig3AlphaTo3", func() {
 		Entry("no resources", noResourcesConfig, noResourcesConfigExp),
 		Entry("basic", basicConfig, basicConfigExp),
 		Entry("complex", complexConfig, complexConfigExp),
+		Entry("no domain", noDomainConfig, noDomainConfigExp),
 	)
 })
 
@@ -68,6 +69,30 @@ version: "3"
 `
 )
 
+const (
+	noDomainConfig = `layout: ansible.sdk.operatorframework.io/v1
+projectName: memcached-operator
+resources:
+- crdVersion: v1
+  group: cache
+  kind: Memcached
+  version: v1alpha1
+version: 3-alpha`
+	noDomainConfigExp = `layout: ansible.sdk.operatorframework.io/v1
+projectName: memcached-operator
+resources:
+- api:
+    crdVersion: v1
+    # TODO(user): Uncomment the below line if this resource's CRD is namespace scoped, else delete it.
+    # namespaced: true
+  # TODO(user): Uncomment the below line if this resource implements a controller, else delete it.
+  # controller: true
+  group: cache
+  kind: Memcached
+  version: v1alpha1
+version: "3"`
+)
+
 const (
 	noResourcesConfig = `domain: example.com
 layout: ansible.sdk.operatorframework.io/v1