Merge ~raharper/curtin:fix/preserve-disk-no-ptable into curtin:master

[Ryan Harper]

block-meta: Don't check the ptable type of a disk with no ptable

When re-using a disk without a partition table we do not need to check
the current partition table.

[Server Team CI bot (server-team-bot)]

PASSED: Continuous integration, rev:4f9731e403347b413af9b722a62861cf32a3dc84
https://jenkins.ubuntu.com/server/job/curtin-ci/34/
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    SUCCESS: https://jenkins.ubuntu.com/server/job/curtin-ci/nodes=metal-arm64/34/
    SUCCESS: https://jenkins.ubuntu.com/server/job/curtin-ci/nodes=metal-ppc64el/34/
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review: Approve (continuous-integration)

[Dan Watkins (oddbloke)]

LGTM, thanks!

review: Approve

diff --git a/curtin/commands/block_meta.py b/curtin/commands/block_meta.py
index 6c145e3..484da41 100644
--- a/curtin/commands/block_meta.py
+++ b/curtin/commands/block_meta.py
@@ -560,7 +560,7 @@ def disk_handler(info, storage_config):
     disk = get_path_to_storage_volume(info.get('id'), storage_config)
     if config.value_as_boolean(info.get('preserve')):
         # Handle preserve flag, verifying if ptable specified in config
-        if ptable != PTABLE_UNSUPPORTED:
+        if ptable and ptable != PTABLE_UNSUPPORTED:
             current_ptable = block.get_part_table_type(disk)
             LOG.debug('disk: current ptable type: %s', current_ptable)
             if current_ptable not in PTABLES_SUPPORTED:
diff --git a/tests/unittests/test_commands_block_meta.py b/tests/unittests/test_commands_block_meta.py
index 7b4f99c..2622a6d 100644
--- a/tests/unittests/test_commands_block_meta.py
+++ b/tests/unittests/test_commands_block_meta.py
@@ -1264,7 +1264,7 @@ class TestDiskHandler(CiTestCase):
         m_getpath.return_value = disk_path
         block_meta.disk_handler(info, storage_config)
         m_getpath.assert_called_with(info['id'], storage_config)
-        self.assertEqual(1, m_block.get_part_table_type.call_count)
+        self.assertEqual(0, m_block.get_part_table_type.call_count)
 
     @patch('curtin.commands.block_meta.block')
     @patch('curtin.commands.block_meta.util')