Fix error message and add fallback test for unset array

botocore/endpoint_provider.py

@@ -601,7 +601,7 @@ def __init__(
         except AttributeError:
             raise EndpointResolutionError(
                 msg=f"Unknown parameter type: {parameter_type}. "
-                "A parameter must be of type string or boolean."
+                "A parameter must be of type string, boolean, or stringarray."
             )
         self.documentation = documentation
         self.builtin = builtIn

tests/unit/data/endpoints/test-cases/array-index.json

@@ -11,6 +11,17 @@
           "url": "https://www.resource.example.com"
         }
       }
+    },
+    {
+      "documentation": "Fallback when array is unset",
+      "params": {
+        "ResourceList": []
+      },
+      "expect": {
+        "endpoint": {
+          "url": "https://www.example.com"
+        }
+      }
     }
   ]
 }

tests/unit/data/endpoints/valid-rules/array-index.json

@@ -32,6 +32,14 @@
         "url": "https://www.{resourceid}.example.com"
       },
       "type": "endpoint"
+    },
+    {
+      "documentation": "Fallback when array is unset",
+      "conditions": [],
+      "endpoint": {
+        "url": "https://www.example.com"
+      },
+      "type": "endpoint"
     }
   ]
 }