[Backport 3.0] [BUG] Agent Framework: Handle model response when toolUse is not accompanied by text

[opensearch-trigger-bot]

Backport 736bf10 from #3755

[codecov]

Codecov Report

Attention: Patch coverage is 85.71429% with 1 line in your changes missing coverage. Please review.

Project coverage is 78.19%. Comparing base (6c5c580) to head (1a2df6c).
Report is 2 commits behind head on 3.0.

Files with missing lines	Patch %	Lines
...nsearch/ml/engine/algorithms/agent/AgentUtils.java	85.71%	0 Missing and 1 partial ⚠️

Additional details and impacted files

@@             Coverage Diff              @@
##                3.0    #3783      +/-   ##
============================================
+ Coverage     78.06%   78.19%   +0.13%     
- Complexity     7111     7117       +6     
============================================
  Files           626      626              
  Lines         32073    32078       +5     
  Branches       3647     3647              
============================================
+ Hits          25037    25083      +46     
+ Misses         5489     5446      -43     
- Partials       1547     1549       +2     

Flag	Coverage Δ
ml-commons	78.19% <85.71%> (+0.13%)	⬆️

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