Skip geometry shader streams > 0 for now. Referencing #27
Handle SemanticName being NULL in D3D11_SO_DECLARATION_ENTRY

* NULL specifies that a gap should be made/skipped in the output data.
* This would crash before but now serialises NULL as "" since that's an
  invalid SemanticName (must be NULL or name a semantic). This will still
  read old logs fine but old programs will not do the right thing on new
  logs.