Fix error when compiling all programs with an unassigned script asset

- Fix error thrown when compiling all programs and there is an UdonSharpProgram asset with no script assigned in the project.

Assets/UdonSharp/Editor/UdonSharpCompiler.cs

@@ -29,7 +29,7 @@ public UdonSharpCompiler(UdonSharpProgramAsset programAsset)
 
         public UdonSharpCompiler(UdonSharpProgramAsset[] programAssets)
         {
-            modules = programAssets.Select(e => new CompilationModule(e)).ToArray();
+            modules = programAssets.Where(e => e.sourceCsScript != null).Select(e => new CompilationModule(e)).ToArray();
         }
 
         public void Compile()