Add nil check and fix type assertion to flattenCloudSchedulerJobHttpTargetHeaders in google_cloud_scheduler_job (#15128)

Author: seitarof

mmv1/templates/terraform/custom_flatten/http_headers.tmpl

@@ -11,23 +11,33 @@
 	limitations under the License.
 */ -}}
 func flatten{{$.GetPrefix}}{{$.TitlelizeProperty}}(v interface{}, d *schema.ResourceData, config *transport_tpg.Config) (interface{}) {
-  var headers = v.(map[string]interface{})
+  if v == nil {
+    return nil
+  }
+  headers, ok := v.(map[string]interface{})
+  if !ok {
+    return nil
+  }
   if v, ok := headers["User-Agent"]; ok {
-    if v.(string) == "AppEngine-Google; (+http://code.google.com/appengine)" {
-      delete(headers, "User-Agent")
-    } else if v.(string) == "Google-Cloud-Scheduler" {
-      delete(headers, "User-Agent")
-    } else {
-      headers["User-Agent"] = strings.TrimSpace(strings.Replace(v.(string), "AppEngine-Google; (+http://code.google.com/appengine)","", -1))
+    if userAgent, ok := v.(string); ok {
+      if userAgent == "AppEngine-Google; (+http://code.google.com/appengine)" {
+        delete(headers, "User-Agent")
+      } else if userAgent == "Google-Cloud-Scheduler" {
+        delete(headers, "User-Agent")
+      } else {
+        headers["User-Agent"] = strings.TrimSpace(strings.Replace(userAgent, "AppEngine-Google; (+http://code.google.com/appengine)","", -1))
+      }
     }
   }
   if v, ok := headers["Content-Type"]; ok {
-    if v.(string) == "application/octet-stream" {
-      delete(headers, "Content-Type")
+    if contentType, ok := v.(string); ok {
+      if contentType == "application/octet-stream" {
+        delete(headers, "Content-Type")
+      }
     }
   }
   r := regexp.MustCompile(`(X-Google-|X-AppEngine-|Content-Length).*`)
-  for key := range headers { 
+  for key := range headers {
     if r.MatchString(key) {
       delete(headers, key)
     }