fix(api): alias api creates index when index name is nill

[rootxrishabh]

Description

When Alias PUT api is called with an empty index name i.e "", the BuildRequest function parses characters after the double quote and before the next / as hostname for the request i.e _alias and request becomes https://localhost:9200/{aliasname} -H _alias -XPUT which actually creates an index.

The fix returns an error when a nil index is passed.

Note -
This is intended to be a temporary fix, however a better fix would be to parse the complete URL i.e instead of creating http request just with path parameters (i.e /<target>/_alias/<alias-name>) at BuildRequest, it would be better to provide the complete URL (i.e https://localhost:9200/<target>/_alias/<alias-name>). This however, requires a redesign of the API system.

Issues Resolved

#650

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[codecov]

Codecov Report

Attention: Patch coverage is 0% with 3 lines in your changes missing coverage. Please review.

Project coverage is 52.82%. Comparing base (06a6dc8) to head (0973c7f).
Report is 99 commits behind head on main.

Files with missing lines	Patch %	Lines
opensearchapi/api_indices-alias.go	0.00%	2 Missing and 1 partial ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #705      +/-   ##
==========================================
- Coverage   57.29%   52.82%   -4.47%     
==========================================
  Files         315      375      +60     
  Lines        9823    11589    +1766     
==========================================
+ Hits         5628     6122     +494     
- Misses       2902     3998    +1096     
- Partials     1293     1469     +176     

Flag	Coverage Δ
integration	52.82% <0.00%> (+1.98%)	⬆️
unit	?

Flags with carried forward coverage won't be shown. Click here to find out more.

Files with missing lines	Coverage Δ
opensearchapi/api_indices-alias.go	97.14% <0.00%> (-2.86%)	⬇️

... and 112 files with indirect coverage changes

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[rootxrishabh]

@dblock @Jakob3xD, this PR is a quick fix to the attached issue. I hope the PR description explains the root cause. If not, I'd be happy to clarify.

[Jakob3xD]

I am not sure yet how I think about it. I generally understand that you want to avoid users of the lib from running into errors or wrong behaviours. But I personally don't think that the lib is responsible for this, as there are many more cases where users can run into such errors or wrong behaviours. Moreover, such special error prevention is harder to handle in code generation. What do you think @dblock ?

[dblock]

This fix could be fine, but it needs a test.