Fix json attachment validation and minimist dependency

[zhouzhuojie]

Description

• Fix [Bug] When attempting to save an invalid JSON blob for a variant attachment, flagr reports success #324
• Bump minimist to be ">=1.2.2"

Motivation and Context

How Has This Been Tested?

UI tests:

Normal Case:

Invalid Case:

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)

Checklist:

• My code follows the code style of this project.
• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I have added tests to cover my changes.
• All new and existing tests passed.

[codecov-io]

Codecov Report

Merging #327 into master will not change coverage by %.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #327   +/-   ##
=======================================
  Coverage   81.91%   81.91%           
=======================================
  Files          26       26           
  Lines        1537     1537           
=======================================
  Hits         1259     1259           
  Misses        209      209           
  Partials       69       69           

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