Fixed NullReferenceException while converting CompilationResult to ShaderBytecode

Previously the implicit conversion operator threw a NullReferenceException because no null check was performed.

When checking for null, compiling a shader directly inside a using clause and returning null if no entry point is given, is possible.

e.g.

```
using (ShaderBytecode geometryShaderByteCode = (!String.IsNullOrEmpty(description.GeometryShaderEntryPoint)) ?
                                                    ShaderBytecode.Compile(uncompiledShaderBytes, description.GeometryShaderEntryPoint, ShaderModel5.GeometryShaderProfile) :
                                                    null)
{
    // ...
}
```