6000.0.29f1 shader error failing builds

### Describe the bug 💬

`Error building Player: Shader error in 'UnityGLTF/PBRGraph': Invalid conditional expression. at Packages/org.khronos.unitygltf/Runtime/Shaders/ShaderGraph/PBRGraph.shadergraph(456)

Compiling Subshader: 0, Pass: Universal Forward Only, Vertex program with EVALUATE_SH_VERTEX _MAIN_LIGHT_SHADOWS_CASCADE _SHADOWS_SOFT_LOW _VOLUME_TRANSMISSION_OFF
Platform defines: SHADER_API_MOBILE UNITY_ENABLE_REFLECTION_BUFFERS UNITY_LIGHTMAP_RGBM_ENCODING UNITY_NO_CUBEMAP_ARRAY UNITY_NO_DXT5nm UNITY_NO_RGBM UNITY_NO_SCREENSPACE_SHADOWS UNITY_PBS_USE_BRDF3 UNITY_PLATFORM_SUPPORTS_DEPTH_FETCH
Disabled keywords: DIRLIGHTMAP_COMBINED DOTS_INSTANCING_ON DYNAMICLIGHTMAP_ON EVALUATE_SH_MIXED FOG_EXP FOG_EXP2 FOG_LINEAR INSTANCING_ON LIGHTMAP_ON LIGHTMAP_SHADOW_MIXING PROBE_VOLUMES_L1 PROBE_VOLUMES_L2 SHADER_API_GLES30 SHADOWS_SHADOWMASK UNITY_ASTC_NORMALMAP_ENCODING UNITY_COLORSPACE_GAMMA UNITY_ENABLE_DETAIL_NORMALMAP UNITY_FRAMEBUFFER_FETCH_AVAILABLE UNITY_HARDWARE_TIER1 UNITY_HARDWARE_TIER2 UNITY_HARDWARE_TIER3 UNITY_LIGHTMAP_DLDR_ENCODING UNITY_LIGHTMAP_FULL_HDR UNITY_LIGHT_PROBE_PROXY_VOLUME UNITY_METAL_SHADOWS_USE_POINT_FILTERING UNITY_PBS_USE_BRDF1 UNITY_PBS_USE_BRDF2 UNITY_PRETRANSFORM_TO_DISPLAY_ORIENTATION UNITY_SPECCUBE_BLENDING UNITY_SPECCUBE_BOX_PROJECTION UNITY_UNIFIED_SHADER_PRECISION_MODEL UNITY_USE_DITHER_MASK_FOR_ALPHABLENDED_SHADOWS UNITY_VIRTUAL_TEXTURING USE_LEGACY_LIGHTMAPS _ADDITIONAL_LIGHTS _ADDITIONAL_LIGHTS_VERTEX _CLEARCOAT_ON _FORWARD_PLUS _IRIDESCENCE_ON _MAIN_LIGHT_SHADOWS _MAIN_LIGHT_SHADOWS_SCREEN _RECEIVE_SHADOWS_OFF _SHEEN_ON _SPECULAR_ON _TEXTURE_TRANSFORM_ON _VOLUME_TRANSMISSION_ANDDISPERSION _VOLUME_TRANSMISSION_ON`

### Steps to reproduce 🔢

`Error building Player: Shader error in 'UnityGLTF/PBRGraph': Invalid conditional expression. at Packages/org.khronos.unitygltf/Runtime/Shaders/ShaderGraph/PBRGraph.shadergraph(456)

Compiling Subshader: 0, Pass: Universal Forward Only, Vertex program with EVALUATE_SH_VERTEX _MAIN_LIGHT_SHADOWS_CASCADE _SHADOWS_SOFT_LOW _VOLUME_TRANSMISSION_OFF
Platform defines: SHADER_API_MOBILE UNITY_ENABLE_REFLECTION_BUFFERS UNITY_LIGHTMAP_RGBM_ENCODING UNITY_NO_CUBEMAP_ARRAY UNITY_NO_DXT5nm UNITY_NO_RGBM UNITY_NO_SCREENSPACE_SHADOWS UNITY_PBS_USE_BRDF3 UNITY_PLATFORM_SUPPORTS_DEPTH_FETCH
Disabled keywords: DIRLIGHTMAP_COMBINED DOTS_INSTANCING_ON DYNAMICLIGHTMAP_ON EVALUATE_SH_MIXED FOG_EXP FOG_EXP2 FOG_LINEAR INSTANCING_ON LIGHTMAP_ON LIGHTMAP_SHADOW_MIXING PROBE_VOLUMES_L1 PROBE_VOLUMES_L2 SHADER_API_GLES30 SHADOWS_SHADOWMASK UNITY_ASTC_NORMALMAP_ENCODING UNITY_COLORSPACE_GAMMA UNITY_ENABLE_DETAIL_NORMALMAP UNITY_FRAMEBUFFER_FETCH_AVAILABLE UNITY_HARDWARE_TIER1 UNITY_HARDWARE_TIER2 UNITY_HARDWARE_TIER3 UNITY_LIGHTMAP_DLDR_ENCODING UNITY_LIGHTMAP_FULL_HDR UNITY_LIGHT_PROBE_PROXY_VOLUME UNITY_METAL_SHADOWS_USE_POINT_FILTERING UNITY_PBS_USE_BRDF1 UNITY_PBS_USE_BRDF2 UNITY_PRETRANSFORM_TO_DISPLAY_ORIENTATION UNITY_SPECCUBE_BLENDING UNITY_SPECCUBE_BOX_PROJECTION UNITY_UNIFIED_SHADER_PRECISION_MODEL UNITY_USE_DITHER_MASK_FOR_ALPHABLENDED_SHADOWS UNITY_VIRTUAL_TEXTURING USE_LEGACY_LIGHTMAPS _ADDITIONAL_LIGHTS _ADDITIONAL_LIGHTS_VERTEX _CLEARCOAT_ON _FORWARD_PLUS _IRIDESCENCE_ON _MAIN_LIGHT_SHADOWS _MAIN_LIGHT_SHADOWS_SCREEN _RECEIVE_SHADOWS_OFF _SHEEN_ON _SPECULAR_ON _TEXTURE_TRANSFORM_ON _VOLUME_TRANSMISSION_ANDDISPERSION _VOLUME_TRANSMISSION_ON`

### Files to reproduce the issue ♻

_No response_

### Editor Version 🎲

6000.0

### Render Pipeline and version

URP

### UnityGLTF Version

2.13

### Operating System 👩‍💻

Linux

### When does this problem happen?

- [ ] Editor Import
- [ ] Runtime Import
- [ ] Editor Export
- [ ] Runtime Export

### Additional Info 📜

_No response_

### Validations 🩹

- [X] I have [searched existing issues](https://github.com/KhronosGroup/UnityGLTF/issues): no issue already exist that reports the same problem.
- [X] I follow the [Code of Conduct](https://www.khronos.org/about/code-of-conduct)
- [X] I provided a [minimal reproducible example](https://stackoverflow.com/help/minimal-reproducible-example), including files when necessary.

Start with Packages/org.khronos.unitygltf/Runtime/Shaders/ShaderGraph/PBRGraph.shadergraph at line 456 and reproduce a Linux player build in Unity 6000.0 with URP and UnityGLTF 2.13. Identify the invalid conditional expression reported by the shader compiler and verify that the build completes without this shader error.

Written by the indexing model from the issue text.