fix(util): Explicitly check for undefined value in getHeaderValues

[gurkohli]

Fix for improbable-eng/grpc-web#1109

getHeaderValues does a truthy check with the value that is returned by headers.get

  const getValue = headers.get(key);
  if (getValue && typeof getValue === "string") {

If getValue is an empty string, it fails this check. The empty string is returned by the function and the logic throws an error when .forEach is done on it. Fix is to correctly verify for non existence of the value and return an empty array in that case

Changes

Updated getHeaderValue to first check for the existence using headers.has. If the value does not exist, return empty array. Removed the truthy check afterwords

Verification

Tested by creating a build, using that build in @improbable/grpc-web client and further using that. The issue improbable-eng/grpc-web#1109 goes away

[gurkohli]

Would you be able to review this or know the best person to review this @MarcusLongmuir ?