Fix validator for elasticsearch strategy

[dimuon]

Description

Related Issues

Fixes #647.

Besides the fix, the PR fixes an issue with generated payload for strategy attribute that prevented the provider from sending correct values for strategy (strategy should be specified as "strategy":{"grow_and_shrink":{}} while it used to be specified as "strategy":{"grow_and_shrink":null}}).

Motivation and Context

How Has This Been Tested?

Types of Changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Refactoring (improves code quality but has no user-facing effect)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation

Readiness 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

[dimuon]

@tobio , can you please have a look again - I changed the implementation a bit.