Handle nop pads correctly

[raganhan]

Nop pads cannot be annotated:

• https://github.com/amzn/ion-tests/blob/master/iontestdata/bad/nopPadWithAnnotations.10n

They are valid inside structs:

• https://github.com/amzn/ion-tests/blob/master/iontestdata/good/nopPadInsideEmptyStructNonZeroSymbolId.10n
• https://github.com/amzn/ion-tests/blob/master/iontestdata/good/nopPadInsideStructWithNopPadThenValueNonZeroSymbolId.10n

Relevant spec excerpt:

NOP padding is valid anywhere a value can be encoded, except for within an annotation wrapper. NOP padding in struct requires additional encoding considerations.