fix empty email on sign-in error (#994)

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<!-- This is an auto-generated comment: release notes by coderabbit.ai
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## Summary by CodeRabbit

* **Bug Fixes**
* Sign-in now rejects empty email values and returns a validation error
instead of accepting them.

* **Tests**
* Added an end-to-end test that verifies signing in with an empty email
returns a schema validation error (HTTP 400).
<!-- end of auto-generated comment: release notes by coderabbit.ai -->

---------

Co-authored-by: Konsti Wohlwend <n2d4xc@gmail.com>

apps/backend/src/app/api/latest/auth/password/sign-in/route.tsx

@@ -20,7 +20,7 @@ export const POST = createSmartRouteHandler({
       tenancy: adaptSchema,
     }).defined(),
     body: yupObject({
-      email: emailSchema.defined(),
+      email: emailSchema.defined().nonEmpty(),
       password: passwordSchema.defined(),
     }).defined(),
   }),

apps/e2e/tests/backend/endpoints/api/v1/auth/password/sign-in.test.ts

@@ -129,3 +129,36 @@ it("should not allow signing in when MFA is required", async ({ expect }) => {
     }
   `);
 });
+
+it("should return a schema error for empty e-mail address", async ({ expect }) => {
+  const response = await niceBackendFetch("/api/v1/auth/password/sign-in", {
+    method: "POST",
+    accessType: "client",
+    body: {
+      email: "",
+      password: "some-password",
+    },
+  });
+  expect(response).toMatchInlineSnapshot(`
+    NiceResponse {
+      "status": 400,
+      "body": {
+        "code": "SCHEMA_ERROR",
+        "details": {
+          "message": deindent\`
+            Request validation failed on POST /api/v1/auth/password/sign-in:
+              - body.email must not be empty
+          \`,
+        },
+        "error": deindent\`
+          Request validation failed on POST /api/v1/auth/password/sign-in:
+            - body.email must not be empty
+        \`,
+      },
+      "headers": Headers {
+        "x-stack-known-error": "SCHEMA_ERROR",
+        <some fields may have been hidden>,
+      },
+    }
+  `);
+});