Fixes the default value for the fix base attribute in urdf converter config (#1367)

# Description

Fixes the default value for the fix base attribute in urdf converter
config

Fixes #1339 


## Type of change

- Bug fix (non-breaking change which fixes an issue)

## Checklist

- [x] I have run the [`pre-commit` checks](https://pre-commit.com/) with
`./isaaclab.sh --format`
- [ ] I have made corresponding changes to the documentation
- [x] My changes generate no new warnings
- [ ] I have added tests that prove my fix is effective or that my
feature works
- [ ] I have updated the changelog and the corresponding version in the
extension's `config/extension.toml` file
- [x] I have added my name to the `CONTRIBUTORS.md` or my name already
exists there

---------
Co-authored-by: Kelly Guo <kellyg@nvidia.com>

source/extensions/omni.isaac.lab/omni/isaac/lab/sim/converters/asset_converter_base.py

@@ -54,6 +54,8 @@ class AssetConverterBase(abc.ABC):
         Raises:
             ValueError: When provided asset file does not exist.
         """
+        # check that the config is valid
+        cfg.validate()
         # check if the asset file exists
         if not check_file_path(cfg.asset_path):
             raise ValueError(f"The asset path does not exist: {cfg.asset_path}")

source/extensions/omni.isaac.lab/omni/isaac/lab/sim/converters/urdf_converter_cfg.py

@@ -30,7 +30,7 @@ class UrdfConverterCfg(AssetConverterBaseCfg):
     """Decompose a convex mesh into smaller pieces for a closer fit. Defaults to False."""
 
     fix_base: bool = MISSING
-    """Create a fix joint to the root/base link. Defaults to True."""
+    """Create a fix joint to the root/base link."""
 
     merge_fixed_joints: bool = False
     """Consolidate links that are connected by fixed joints. Defaults to False."""