Fix undefined dateOfBirth (#86)

# Fix undefined dateOfBirth

## ♻️ Current situation & Problem
If `dateOfBirth` is `undefined`, Firestore throws an error.


## ⚙️ Release Notes 
* Fix undefined dateOfBirth


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routes/~_dashboard/~patients/~$id/~index.tsx

@@ -109,7 +109,7 @@ const PatientPage = () => {
       invitationCode: form.invitationCode,
       clinician: form.clinician,
       organization: clinician.organization,
-      dateOfBirth: form.dateOfBirth?.toISOString(),
+      dateOfBirth: form.dateOfBirth?.toISOString() ?? null,
     }
     if (resourceType === 'user') {
       await callables.updateUserInformation({