boot: handle small disk case

The BIOS boot plan check assumes that there is at least one result in
the list from gaps.parts_and_gaps(), but that isn't the case if the disk
is small.

Showed up in VM testing using cloud-localds to seed the autoinstall
file.

subiquity/common/filesystem/boot.py

@@ -154,6 +154,7 @@ def get_boot_device_plan_bios(device) -> Optional[MakeBootDevicePlan]:
     if device.ptable == 'msdos':
         return attr_plan
     pgs = gaps.parts_and_gaps(device)
+    if len(pgs) > 0:
         if isinstance(pgs[0], Partition) and pgs[0].flag == "bios_grub":
             return attr_plan
 

subiquity/common/filesystem/tests/test_manipulator.py

@@ -457,6 +457,14 @@ class TestFilesystemManipulator(unittest.TestCase):
             preserve=True)
         self.assertFalse(boot.can_be_boot_device(disk))
 
+    @parameterized.expand([(1,), (2,)])
+    def test_no_add_boot_BIOS_gpt_teeny_disk(self, version):
+        # Showed up in VM testing - the ISO created cloud-localds shows up as a
+        # potential install target, and weird things happen.
+        manipulator = make_manipulator(Bootloader.BIOS, version)
+        disk = make_disk(manipulator.model, ptable='gpt', size=1 << 20)
+        self.assertFalse(boot.can_be_boot_device(disk))
+
     @parameterized.expand([(1,), (2,)])
     def test_add_boot_BIOS_msdos(self, version):
         manipulator = make_manipulator(Bootloader.BIOS, version)