chore: fix all integ tests (#22821)

Currently, many (all?) our integ tests fail _deployment_ with

```
Error: Resolution error: Resolution error: Resolution error: Resolution error:
Context value '@aws-cdk/core:target-partitions' should be a list of strings,
got: '@aws-cdk/core:target-partitions'.
```

This is a result of #22771, which introduced `[TARGET_PARTITIONS]: undefined,` to the `this.getContext()` method in the integ runner.
However, the integ runner stringifies all context in a few places like [here](https://github.com/aws/aws-cdk/blob/main/packages/@aws-cdk/integ-runner/lib/runner/runner-base.ts#L335). That makes the value of `TARGET_PARTITIONS = 'undefined'`.

The error message also fails to provide a decent understanding of what's going on, due to a typo that this PR also fixes.
This fix fixes the integ test I was trying to run.

----

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### Adding new Unconventional Dependencies:

* [ ] This PR adds new unconventional dependencies following the process described [here](https://github.com/aws/aws-cdk/blob/main/CONTRIBUTING.md/#adding-new-unconventional-dependencies)

### New Features

* [ ] Have you added the new feature to an [integration test](https://github.com/aws/aws-cdk/blob/main/INTEGRATION_TESTS.md)?
	* [ ] Did you use `yarn integ` to deploy the infrastructure and generate the snapshot (i.e. `yarn integ` without `--dry-run`)?

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

Author: kaizencc

packages/@aws-cdk/core/lib/stack.ts

@@ -870,8 +870,8 @@ export class Stack extends Construct implements ITaggable {
     }
 
     const partitions = Node.of(this).tryGetContext(cxapi.TARGET_PARTITIONS);
-    if (partitions !== undefined && !Array.isArray(partitions)) {
-      throw new Error(`Context value '${cxapi.TARGET_PARTITIONS}' should be a list of strings, got: ${JSON.stringify(cxapi.TARGET_PARTITIONS)}`);
+    if (partitions !== undefined && partitions !== 'undefined' && !Array.isArray(partitions)) {
+      throw new Error(`Context value '${cxapi.TARGET_PARTITIONS}' should be a list of strings, got: ${JSON.stringify(partitions)}`);
     }
 
     const lookupMap = partitions ? RegionInfo.limitedRegionMap(factName, partitions) : RegionInfo.regionMap(factName);