[Backport to 2.x] allow input null for text docs input (#1401)

[ylwu-amzn]

Description

Backport #1401 to 2.x

Issues Resolved

[List any issues this PR will resolve]

Check List

• New functionality includes testing.
  • All tests pass
• New functionality has been documented.
  • New functionality has javadoc added
• Commits are signed per the DCO using --signoff

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[codecov]

Codecov Report

Merging #1402 (aa91556) into 2.x (5b59e3e) will increase coverage by 0.04%.
Report is 1 commits behind head on 2.x.
The diff coverage is 100.00%.

@@             Coverage Diff              @@
##                2.x    #1402      +/-   ##
============================================
+ Coverage     78.23%   78.28%   +0.04%     
- Complexity     2301     2304       +3     
============================================
  Files           195      195              
  Lines          9370     9370              
  Branches        917      917              
============================================
+ Hits           7331     7335       +4     
+ Misses         1624     1621       -3     
+ Partials        415      414       -1     

Flag	Coverage Δ
ml-commons	78.28% <100.00%> (+0.04%)	⬆️

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Files	Coverage Δ
...h/ml/action/deploy/TransportDeployModelAction.java	64.97% <100.00%> (ø)

... and 1 file with indirect coverage changes