Skip the encoder for null optional elements that write nothing

[veewee]

Q	A
Type	improvement
BC Break	no
Fixed issues

Stacked on #79, so it targets that branch until it is merged.

Null optional elements on encode

A null value on an optional element that is not nillable goes through ErrorHandlingEncoder and OptionalElementEncoder, only to come back as an empty string that writes nothing. On sparse payloads that is most of the elements, and it is the largest cost left in sparse encoding.

ObjectAccess::build now asks the encoder of every child element once per type what a null value does, and keeps the answer in one onNull map. The answer is a NullValueHandling case: Write lets the encoder write it, Skip means the encoder writes nothing for null, and SkipWhenAbsent leaves the element out when the property is absent and writes xsi:nil when it is set to null. Encoders answer through a new Feature\NullValueAware interface. OptionalElementEncoder answers Skip for a nullable element that is not nillable, the same rule its iso uses, SkipWhenAbsent for an optional nillable element, and Write otherwise. ErrorHandlingEncoder forwards the question. Attributes and the _ text value always get Write, and a custom encoder lens never counts as absent, as in #79. ObjectEncoder::to has one null check that reads this map, and replaces the absent check from #79 as well.

Same output

This does not depend on how the encoders are nested. Any encoder that does not implement the interface keeps its own null handling, so a custom encoder that writes something for null is never skipped, and a custom encoder can opt in. Custom encoder lenses still run before the check. One thing does change compared to #79: a custom OptionalAware encoder that is not wrapped in OptionalElementEncoder no longer has its absent optional nillable elements left out unless it implements NullValueAware, because only the encoder knows what it writes for null.

One output detail changes: an object whose elements are all null encoded to <a></a> and now encodes to <a/>. Both are the same empty element, and an object whose nillable optional elements are all absent already gets <a/> since #79. The tests pin the output byte for byte for all null, all absent, leading and trailing nulls, a nested empty object and a custom encoder. The encoded requests and decoded results of the 3MB workloads have the same sha1 before and after, because none of their objects is empty.

Speed

Base is absent-optional-elements (#79). Medians over three interleaved rounds on the same machine, with ext-soap as the noise reference.

Workload	#79	This PR
3MB encode sparse, optional elements without nillable (stdClass)	1.46 s	0.99 s
3MB encode sparse, optional elements without nillable (classmap)	1.60 s	1.13 s
3MB encode dense (stdClass)	0.72 s	0.71 s
3MB encode sparse, absent nillable elements (stdClass)	1.05 s	1.06 s
3MB decode sparse (stdClass)	0.50 s	0.49 s
EncodeSparseBench sparse x500	15.5 ms	15.5 ms
EncodeSparseBench all null x500	25.6 ms	26.6 ms
EncodeBench literal x500	33.9 ms	33.6 ms
ext-soap encode literal x500	3.79 ms	3.66 ms

The first two rows are the target and drop by about a third. EncodeSparseBench uses nillable elements, which this change does not skip, so those rows and the others stay within the run to run spread.

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