Prevent writing empty annotations sequences

[zslayton]

Prior to this change, when a ValueWriter explicitly called with_annotations() and provided an empty array, the writer would write an empty annotations sequence which is not legal. This change detects that case and skips writing any sequence at all. It also adds a has_annotations() convenience method to the LazyRawValue trait.

By submitting this pull request, I confirm that my contribution is made under the terms of the Apache 2.0 license.

[popematt]

Can you add test cases to verify that the other encodings do not have the same issue? (Or do they already exist?)

[zslayton]

Can you add test cases to verify that the other encodings do not have the same issue? (Or do they already exist?)

The Binary 1.1 writer already had a test for this:

ion-rust/src/lazy/encoder/binary/v1_1/value_writer.rs

Lines 2632 to 2642 in 80088b4

	// Explicitly empty annotations set with a type hint the compiler can use in a generic context.
	const NO_ANNOTATIONS: [SymbolId; 0] = [];
	// === Symbol ID annotations ===
	case(
	0.annotated_with(NO_ANNOTATIONS),
	&[
	// Integer 0
	0x50,
	],
	)?;

I'll add one for the text writers though.