fix(clients): handle empty xml tags

[AllanZhengYP]

Issue

Resolves: #3585
Follow up to: #885 #914 #919 #955

Description

The underlying xml parser used by the SDK(fast-xml-parser) parses the self-closing tags(e.g. <Filter/>) to empty string(e.g. { "Filter": "" }). The mentioned PRs(#914 #919 #955) adds early return to the parser. If the parsed value of the member is empty string, it will return default empty value. If the member is a Map, then the empty value is {}; If the member is a Collection(List), then the empty value is [].

However the generated code has 2 issues:

1. The early if statement was not effective as the later if statement will still attempt to parse the subordinary shape out of "". For example

   if (output.Tag === "") {
     contents.Tags = []; // <== this is overwritten
   }
   if (output["Tag"] !== undefined) { 
     contents.Tags = deserializeAws_restXmlTagSet(__getArrayIfSingleItem(output["Tag"]), context);
   }

   fix:

   if (output.Tag === "") {
     contents.Tags = [];
   }
   else if (output["Tag"] !== undefined) { 
     contents.Tags = deserializeAws_restXmlTagSet(__getArrayIfSingleItem(output["Tag"]), context);
   }

2. The Union shape as a special type of structure that requires exactly one member, should also be skipped if serialized in empty tag, just like Map and Collection.

This change fixes both issues.

Testing

Unit-test

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