fix: empty strings in enums when type [string, null] (#1200)

* test: add failing tests for enums with type [string, null]

* fix: empty strings in enums when type [string, null]

* test: change polyphormism test examples

* refactor: remove outdated comment

* chore: add changeset

* chore: update autogenerated examples

Author: toomuchdesign

.changeset/shy-turtles-taste.md

@@ -0,0 +1,5 @@
+---
+"openapi-typescript": patch
+---
+
+Remove unexpected empty string in generated nullable polymophic enum types (["string", "null"])

packages/openapi-typescript/examples/github-api-next.ts

@@ -3174,7 +3174,7 @@ export interface components {
      * @description **Required when the `state` is `resolved`.** The reason for resolving the alert. 
      * @enum {string|null}
      */
-    "secret-scanning-alert-resolution": "" | "false_positive" | "wont_fix" | "revoked" | "used_in_tests" | null;
+    "secret-scanning-alert-resolution": null | "false_positive" | "wont_fix" | "revoked" | "used_in_tests";
     /**
      * Minimal Repository 
      * @description Minimal Repository

packages/openapi-typescript/examples/github-api.ts

@@ -51,13 +51,13 @@ export function defaultSchemaObjectTransform(schemaObject: SchemaObject | Refere
   if (schemaObject.enum) {
     let items = schemaObject.enum as any[];
     if ("type" in schemaObject) {
-      if (schemaObject.type === "string" || (Array.isArray(schemaObject.type) && schemaObject.type.includes("string"))) items = items.map((t) => escStr(t || "")); // empty/missing values are empty strings
+      if (schemaObject.type === "string" || (Array.isArray(schemaObject.type) && schemaObject.type.includes("string"))) items = items.map((t) => escStr(t));
     }
     // if no type, assume "string"
     else {
       items = items.map((t) => escStr(t || ""));
     }
-    return tsUnionOf(...items, ...(schemaObject.nullable || ("type" in schemaObject && Array.isArray(schemaObject.type) && schemaObject.type.includes("null")) ? ["null"] : []));
+    return tsUnionOf(...items, ...(schemaObject.nullable ? ["null"] : []));
   }
 
   // oneOf (no discriminator)

packages/openapi-typescript/examples/octokit-ghes-3.6-diff-to-api.ts

@@ -278,9 +278,19 @@ describe("Schema Object", () => {
         expect(generated).toBe("OneOf<[string, boolean, number, null]>");
       });
 
-      test("enum + polymorphism + nullable", () => {
-        const generated = transformSchemaObject({ type: ["string", "null"], enum: ["", "false positive", "won't fix", "used in tests"] }, options);
-        expect(generated).toBe(`"" | "false positive" | "won't fix" | "used in tests" | null`);
+      test("enum + polymorphism + nullable 1", () => {
+        const generated = transformSchemaObject({ type: ["string", "null"], enum: ["blue", "green", "yellow"] }, options);
+        expect(generated).toBe(`"blue" | "green" | "yellow"`);
+      });
+
+      test("enum + polymorphism + nullable 2", () => {
+        const generated = transformSchemaObject({ type: ["string", "null"], enum: ["", "blue", "green", "yellow"] }, options);
+        expect(generated).toBe(`"" | "blue" | "green" | "yellow"`);
+      });
+
+      test("enum + polymorphism + nullable 3", () => {
+        const generated = transformSchemaObject({ type: ["string", "null"], enum: [null, "blue", "green", "yellow"] }, options);
+        expect(generated).toBe(`null | "blue" | "green" | "yellow"`);
       });
     });
   });