SDKS-4714: Use empty request body in self-service session endpoint

[witrisna]

JIRA Ticket

SDKS-4714

Description

Remove the EMPTY constant from OkHttp's RequestBody and has been replaced with "".toRequestBody().

Definition of Done Checklist:

• Coded to standards.
• Ensure backward compatibility.
• API reference docs is created or updated, if applicable.
• Unit tests are written or updated.
• Integration tests are written, if applicable.

[spetrov]

LGTM, but please update the copyright header...

[codecov]

Codecov Report

❌ Patch coverage is 50.00000% with 1 line in your changes missing coverage. Please review.
✅ Project coverage is 65.75%. Comparing base (13891a1) to head (16b5d06).
⚠️ Report is 1 commits behind head on develop.

Files with missing lines	Patch %	Lines
.../org/forgerock/android/auth/selfservice/Session.kt	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             develop     #510   +/-   ##
==========================================
  Coverage      65.75%   65.75%           
  Complexity      1764     1764           
==========================================
  Files            264      264           
  Lines           8658     8658           
  Branches         969      969           
==========================================
  Hits            5693     5693           
  Misses          2534     2534           
  Partials         431      431           

Flag	Coverage Δ
unit-tests	65.75% <50.00%> (ø)

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