fix: do not append to undefined/null header values

[Amplifiyer]

Issue #, if available: N/A

Description of changes:
Avoid appending to user agent header when the existing value is undefined or null to avoid getting header values as "undefined myUserAgent"

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[codecov-io]

Codecov Report

Merging #388 into master will decrease coverage by 7.98%.
The diff coverage is 100%.

@@            Coverage Diff             @@
##           master     #388      +/-   ##
==========================================
- Coverage     100%   92.01%   -7.99%     
==========================================
  Files           5      122     +117     
  Lines          72     3170    +3098     
  Branches       18      714     +696     
==========================================
+ Hits           72     2917    +2845     
- Misses          0      253     +253

Impacted Files	Coverage Δ
packages/util-user-agent-browser/src/index.ts	90.9% <100%> (ø)
...ackages/protocol-json-rpc/src/JsonRpcSerializer.ts	100% <0%> (ø)
packages/signature-v4/src/constants.ts	100% <0%> (ø)
packages/xml-builder/src/XmlText.ts	100% <0%> (ø)
packages/json-builder/src/operations.fixtures.ts	100% <0%> (ø)
packages/url-parser-node/src/index.ts	72.72% <0%> (ø)
packages/protocol-json-rpc/src/JsonRpcParser.ts	100% <0%> (ø)
...dleware-sdk-glacier/src/add-glacier-api-version.ts	100% <0%> (ø)
...ackages/http-serialization/src/requests.fixture.ts	100% <0%> (ø)
... and 108 more

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