fix(core): the string 'undefined' is recognized as a valid partition when looking up for fact values (#23023)

When the partition value is the literal string 'undefined' (because it was stringified from the context), it should be treated as if it is actually `undefined`. When looking up a fact value, the two values are already considered the same in one code path, but are then treated differently in the immediate logic. This is causing a new integration test to fail.

This commit makes the logic consistent.


----

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*By submitting this pull request, I confirm that my contribution is made under the terms of the Apache-2.0 license*

Author: huyphan

Diff for: packages/@aws-cdk/core/lib/stack.ts

@@ -960,7 +960,10 @@ export class Stack extends Construct implements ITaggable {
       throw new Error(`Context value '${cxapi.TARGET_PARTITIONS}' should be a list of strings, got: ${JSON.stringify(partitions)}`);
     }
 
-    const lookupMap = partitions ? RegionInfo.limitedRegionMap(factName, partitions) : RegionInfo.regionMap(factName);
+    const lookupMap =
+      partitions !== undefined && partitions !== 'undefined'
+        ? RegionInfo.limitedRegionMap(factName, partitions)
+        : RegionInfo.regionMap(factName);
 
     return deployTimeLookup(this, factName, lookupMap, defaultValue);
   }

Diff for: packages/@aws-cdk/core/test/stack.test.ts

@@ -1719,6 +1719,31 @@ describe('regionalFact', () => {
     expect(stack.regionalFact('MyFact')).toEqual('x.amazonaws.com');
   });
 
+  test('regional facts use the global lookup map if partition is the literal string of "undefined"', () => {
+    const stack = new Stack();
+    Node.of(stack).setContext(cxapi.TARGET_PARTITIONS, 'undefined');
+    new CfnOutput(stack, 'TheFact', {
+      value: stack.regionalFact('WeirdFact'),
+    });
+
+    expect(toCloudFormation(stack)).toEqual({
+      Mappings: {
+        WeirdFactMap: {
+          'eu-west-1': { value: 'otherformat' },
+          'us-east-1': { value: 'oneformat' },
+        },
+      },
+      Outputs: {
+        TheFact: {
+          Value: {
+            'Fn::FindInMap': ['WeirdFactMap', { Ref: 'AWS::Region' }, 'value'],
+          },
+        },
+      },
+    });
+
+  });
+
   test('regional facts generate a mapping if necessary', () => {
     const stack = new Stack();
     new CfnOutput(stack, 'TheFact', {