URP: Ensure ScriptableRendererFeatures are compiled before validating them.

This PR add an extra check to `ScriptableRendererData.OnValidate` to ensure we only validate `ScriptableRendererFeatures` when all scripts have been compiled. Not doing so can lead to errors being thrown to the console during import (see attached Jira ticket).

Jira: https://jira.unity3d.com/browse/UUM-58944

Author: amsXYZ

Packages/com.unity.render-pipelines.universal/Runtime/ScriptableRendererData.cs

@@ -59,7 +59,9 @@ protected virtual void OnValidate()
         {
             SetDirty();
 #if UNITY_EDITOR
-            if (m_RendererFeatures.Contains(null))
+            // Only validate ScriptableRendererFeatures when all scripts have finished compiling (to avoid false-negatives
+            // when ScriptableRendererFeatures haven't been compiled before this check).
+            if (!EditorApplication.isCompiling && m_RendererFeatures.Contains(null))
                 ValidateRendererFeatures();
 #endif
         }