fix(stateMachine): handle plain string Fn::Sub when useExactVersion is true (#744)

When a state machine uses pseudo-parameters (#{...}) but no lambda
function references, Fn::Sub is set to a plain string rather than
an array. The useExactVersion code path then tried to assign to
index [1] of that string, throwing a TypeError in strict mode.

Fix by converting the plain string form to the array form
[template, {}] before applying the version conversion.

Closes #488

Co-authored-by: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: VirtueMe

lib/deploy/stepFunctions/compileStateMachines.js

@@ -167,6 +167,9 @@ module.exports = {
           }
         }
         if (stateMachineObj.useExactVersion === true && DefinitionString['Fn::Sub']) {
+          if (!Array.isArray(DefinitionString['Fn::Sub'])) {
+            DefinitionString['Fn::Sub'] = [DefinitionString['Fn::Sub'], {}];
+          }
           const params = DefinitionString['Fn::Sub'][1];
           const f = convertToFunctionVersion.bind(this);
           const converted = _.mapValues(params, f);

lib/deploy/stepFunctions/compileStateMachines.test.js

@@ -1277,6 +1277,42 @@ describe('#compileStateMachines', () => {
         .to.equal('arn:aws:lambda:us-east-1:1234567890:function:world');
     });
 
+    it('should not throw when useExactVersion is true and definition uses pseudo-parameters but no lambda refs', () => {
+      serverless.service.stepFunctions = {
+        stateMachines: {
+          myStateMachine1: {
+            id: 'Test',
+            useExactVersion: true,
+            definition: {
+              StartAt: 'BatchJob',
+              States: {
+                BatchJob: {
+                  Type: 'Task',
+                  Resource: 'arn:aws:states:::batch:submitJob.sync',
+                  Parameters: {
+                    JobName: 'MyJob',
+                    JobDefinition: '#{SlsJobDefinition}',
+                    JobQueue: '#{SlsJobQueue}',
+                  },
+                  End: true,
+                },
+              },
+            },
+          },
+        },
+      };
+
+      expect(() => serverlessStepFunctions.compileStateMachines()).to.not.throw();
+
+      const stateMachine = serverlessStepFunctions.serverless.service
+        .provider.compiledCloudFormationTemplate.Resources.Test;
+      const { 'Fn::Sub': fnSub } = stateMachine.Properties.DefinitionString;
+
+      expect(Array.isArray(fnSub)).to.equal(true);
+      expect(fnSub[0]).to.include('${SlsJobDefinition}');
+      expect(fnSub[0]).to.include('${SlsJobQueue}');
+    });
+
     it('should do nothing if there are no ref to lambda functions, even if useExactVersion is true', () => {
       serverless.service.stepFunctions = {
         stateMachines: {