Add DbContext null check to FieldInterceptor.cs

In some cases (outlined under "caveat" here, http://entityframework.codeplex.com/wikipage?title=Interception) EntityFramework interceptors may not have any DbContexts associated with them.  The current code will cause a null reference exception.  This can be replicated by using a profiling tool such as Glimpse.EF6 with a project using EntityFramework.Filters.  This PR adds a null check for context before continuing with filter execution.