URDF gazebo material tags incorrectly translated

## Environment
* OS Version: Ubuntu 20.04
* Source or binary build? Binary, apt, 9.4.0-1

## Description
* Expected behavior:
Gazebo material tags in the URDF that are already valid SDF material tags should be translated unchanged.
* Actual behavior:
The translation process breaks the material tag by attempting to convert it to a reference to a Gazebo material script.

## Steps to reproduce

1. Create a URDF including a gazebo material tag. Example:
```




















0 0 1 1
0 0 1 1
0 0 1 1



```
2. Convert the URDF file to SDF
```
ign sdf -p example.urdf
```

## Output
The generated SDF with the now broken material tag.
```

0 0 0 0 -0 0
1

1
0
0
1
0
1



0 0 0 0 -0 0


1 1 1












0 0 0 0 -0 0


1 1 1




<name>ambient</name>
<uri>file://media/materials/scripts/gazebo.material</uri>



1

```

Reproduce the issue with the inline URDF example and run `ign sdf -p example.urdf` to inspect the URDF-to-SDF material translation. Trace the handling of the `` tags and verify that valid SDF ambient, diffuse, and specular elements remain unchanged instead of becoming a script reference.

Written by the indexing model from the issue text.