Fix error when downloading spool files with default encoding

[t1m0thyj]

What It Does

Fixes z/OSMF error when DownloadJobs.downloadSpoolContentCommon is called and encoding property is undefined.

Also corrects the behavior of Get.dataSet and Get.USSFile methods so that they return an empty buffer instead of null for empty files.

How to Test

Run the following system test files and ensure that all tests pass:

• DownloadJobs.system.test.ts
• cli.dir.upload.dtu.system.test.ts
• cli.files.upload.stds.system.test.ts

Review Checklist I certify that I have:

• [x] tested my changes
• [x] added/updated automated tests
• [x] updated the changelog
• [x] followed the contribution guidelines

Additional Comments

[codecov[bot]]

Codecov Report

All modified and coverable lines are covered by tests :white_check_mark:

Project coverage is 91.14%. Comparing base (8a7f816) to head (96c1d2b). Report is 1 commits behind head on master.

Additional details and impacted files

```diff @@ Coverage Diff @@ ## master #2173 +/- ## ======================================= Coverage 91.14% 91.14% ======================================= Files 637 637 Lines 19064 19064 Branches 4013 4015 +2 ======================================= Hits 17376 17376 Misses 1687 1687 Partials 1 1 ```

:umbrella: View full report in Codecov by Sentry.
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[sonarcloud[bot]]

Quality Gate passed

Issues
0 New issues
0 Accepted issues

Measures
0 Security Hotspots
100.0% Coverage on New Code
0.0% Duplication on New Code

See analysis details on SonarCloud

[github-actions[bot]]

Release succeeded for the master branch. :tada:

The following packages have been published:

• npm: @zowe/zos-files-for-zowe-sdk@7.26.1
• npm: @zowe/zos-workflows-for-zowe-sdk@7.26.1
• npm: @zowe/zos-jobs-for-zowe-sdk@7.26.1
• npm: @zowe/cli@7.26.1

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