fix(assertions): 'pattern.indexOf' is not a function (aws#19009)

In some cases, some CloudFormation templates lead this function to
fail. Without looking at the exact templates it's hard to see why,
but we can at least be more vigilant against the occurence and check
types a bit stronger.


----

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

packages/@aws-cdk/assertions/lib/private/cyclic.ts

@@ -70,11 +70,16 @@ function findExpressionDependencies(obj: any): Set<string> {
       } else if (keys.length === 1 && keys[0] === 'Fn::Sub') {
         const argument = x[keys[0]];
         const pattern = Array.isArray(argument) ? argument[0] : argument;
-        for (const logId of logicalIdsInSubString(pattern)) {
-          ret.add(logId);
+
+        // pattern should always be a string, but we've encountered some cases in which
+        // it isn't. Better safeguard.
+        if (typeof pattern === 'string') {
+          for (const logId of logicalIdsInSubString(pattern)) {
+            ret.add(logId);
+          }
         }
         const contextDict = Array.isArray(argument) ? argument[1] : undefined;
-        if (contextDict) {
+        if (contextDict && typeof contextDict === 'object') {
           Object.values(contextDict).forEach(recurse);
         }
       } else {