fix: Fail gracefully when AutoPublishAlias is an empty string (#2425)

Co-authored-by: Jacob Fuss <[email protected]>

Author: jfuss

samtranslator/model/sam_resources.py

@@ -878,7 +878,7 @@ def _validate_deployment_preference_and_add_update_policy(
         )
 
         if deployment_preference_collection.get(self.logical_id).enabled:
-            if self.AutoPublishAlias is None:
+            if not self.AutoPublishAlias:
                 raise InvalidResourceException(
                     self.logical_id, "'DeploymentPreference' requires AutoPublishAlias property to be specified."
                 )

tests/translator/input/error_auto_publish_alias_empty_string.yaml

@@ -0,0 +1,10 @@
+Resources:
+  MinimalFunction:
+    Type: 'AWS::Serverless::Function'
+    Properties:
+      CodeUri: s3://sam-demo-bucket/hello.zip
+      Handler: hello.handler
+      Runtime: python2.7
+      AutoPublishAlias: ''
+      DeploymentPreference:
+        Type: AllAtOnce

tests/translator/output/error_auto_publish_alias_empty_string.json

@@ -0,0 +1,3 @@
+{
+    "errorMessage": "Invalid Serverless Application Specification document. Number of errors found: 1. Resource with id [MinimalFunction] is invalid. 'DeploymentPreference' requires AutoPublishAlias property to be specified."
+}