What were you trying to do that didn't work?

What is the impact of this issue to you?

Please provide the package NVR for which the bug is seen:

How reproducible is this bug?:

Steps to reproduce

Expected results

Should return `NULL` (None in Python) or raise a `GError` because:

• Empty string is not a valid DM map name
• There is no DM device with an empty name
• "mapper" is not a valid DM node name for an empty map

Actual results

Returns `"mapper"`, which is:

• Incorrect - there is no DM node named "mapper"
• Misleading - suggests empty string is a valid map name
• Derived from the `/dev/mapper` directory path, not a real device

The function `bd_dm_node_from_name()` returns the string `"mapper"` when called with an empty string `""` as the map name, instead of returning `NULL` or raising an error. This violates the API contract and can lead to incorrect behavior in calling code.

```python
import gi
gi.require_version("BlockDev", "3.0")
from gi.repository import BlockDev as bdbd.init(None, None)# Call with empty string
result = bd.dm_node_from_name("")print(f"Result: {repr(result)}")  # Prints: "mapper" (incorrect!)
```

libblockdev-3.4.0-2.el10.x86_64