Make dailytestrollup branch nullable

We've been seeing a bunch of errors related to this field not being nullable.
This is because it's taking the branch from the commit, which _can_ be nullable.

See: [error](https://codecov.sentry.io/share/issue/478a3e60158547b69afa0b9959822237/)

shared/django_apps/reports/migrations/0025_alter_dailytestrollup_branch.py

@@ -0,0 +1,26 @@
+# Generated by Django 4.2.15 on 2024-09-20 20:21
+
+from django.db import migrations, models
+
+"""
+BEGIN;
+--
+-- Alter field branch on dailytestrollup
+--
+ALTER TABLE "reports_dailytestrollups" ALTER COLUMN "branch" DROP NOT NULL;
+COMMIT;
+"""
+
+
+class Migration(migrations.Migration):
+    dependencies = [
+        ("reports", "0024_auto_20240917_2121"),
+    ]
+
+    operations = [
+        migrations.AlterField(
+            model_name="dailytestrollup",
+            name="branch",
+            field=models.TextField(null=True),
+        ),
+    ]

shared/django_apps/reports/models.py

@@ -415,7 +415,7 @@ class PartitioningMeta:
     )
     date = models.DateField()
     repoid = models.IntegerField()
-    branch = models.TextField()
+    branch = models.TextField(null=True)
 
     fail_count = models.IntegerField()
     flaky_fail_count = models.IntegerField()