Accept null as concurrency property

This makes sure we are accepting null as well so users can still set a default value
for concurrency groups but also accept null values.

src/__snapshots__/workflow.spec.ts.snap

@@ -65,6 +65,23 @@ jobs:
 "
 `;
 
+exports[`Workflow > allows creating jobs with concurrency set to null 1`] = `
+"# Workflow automatically generated by gat
+# DO NOT CHANGE THIS FILE MANUALLY
+
+name: Without concurrency
+on:
+  push: null
+jobs:
+  job1:
+    runs-on: ubuntu-22.04
+    timeout-minutes: 15
+    steps:
+      - name: Do something
+        run: exit 0
+"
+`;
+
 exports[`Workflow > allows custom types in a workflow 1`] = `
 "# Workflow automatically generated by gat
 # DO NOT CHANGE THIS FILE MANUALLY

src/job.ts

@@ -1,7 +1,7 @@
-export interface ConcurrencyGroup {
+export type ConcurrencyGroup = {
   groupSuffix: string;
   cancelPrevious: boolean;
-}
+};
 
 export interface Matrix {
   elements: Array<{ id: string; options: Array<string | number | boolean> }>;
@@ -25,7 +25,7 @@ export interface JobOptions<Step, RunnerDefinition, Name> {
   dependsOn?: Array<Name>;
   services?: Record<string, Service>;
   env?: Record<string, string>;
-  concurrency?: ConcurrencyGroup;
+  concurrency?: ConcurrencyGroup | null;
   matrix?: Matrix | string;
   steps: Step[];
   outputs?: Record<string, string>;

src/workflow.spec.ts

@@ -270,4 +270,20 @@ exit 0`,
       });
     expect(await workflow.compile()).toMatchSnapshot();
   });
+
+  it("allows creating jobs with concurrency set to null", async () => {
+    const workflow = new Workflow("Without concurrency")
+      .on("push")
+      .addJob("job1", {
+        concurrency: null,
+        steps: [
+          {
+            name: "Do something",
+            run: "exit 0",
+          },
+        ],
+      });
+
+    expect(await workflow.compile()).toMatchSnapshot();
+  });
 });

src/workflow.ts

@@ -79,7 +79,7 @@ export class Workflow<
   jobs: Array<Job<JobStep, Runner, JobName>>;
   defaultOptions: DefaultOptions | null;
   env: EnvVar[];
-  concurrencyGroup?: ConcurrencyGroup;
+  concurrencyGroup?: ConcurrencyGroup | null;
 
   constructor(public name: string) {
     this.events = [];
@@ -111,7 +111,7 @@ export class Workflow<
     return this;
   }
 
-  setConcurrencyGroup(concurrencyGroup: ConcurrencyGroup) {
+  setConcurrencyGroup(concurrencyGroup: ConcurrencyGroup | null) {
     this.concurrencyGroup = concurrencyGroup;
     return this;
   }