Harden flipper feature.get access

What does this MR do?

Harden flipper feature.get access

For ab_feature_enabled makes sure we don't get a runtime error if flipper implementation for get changes

Closes #220008 (closed)

The Feature.get always returns an object no matter what, so even if flipper_feature wasn't needed - but could fail if flipper changed that behavior.

pry(main)> ::Feature.get(:non_existing_ff).gate_values[:percentage_of_actors]
  Feature::FlipperGate Load (0.3ms)  SELECT "feature_gates".* FROM "feature_gates" WHERE "feature_gates"."feature_key" = 'non_existing_ff'
=> 0
pry(main)> ::Feature.get(:non_existing_ff)
=> #<Flipper::Feature:47281543805920 name=:non_existing_ff, state=:off, enabled_gate_names=[], adapter=:memoizable>
pry(main)> ::Feature.get(:non_existing_ff).gate_values[:percentage_of_actors]
=> 0

Does this MR meet the acceptance criteria?

Conformity

• Changelog entry
• Documentation (if required)
• Code review guidelines
• Merge request performance guidelines
• Style guides
• Database guides
• Separation of EE specific content

Availability and Testing

• Review and add/update tests for this feature/bug. Consider all test levels. See the Test Planning Process.
• Tested in all supported browsers
• Informed Infrastructure department of a default or new setting change, if applicable per definition of done

Security

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• Label as security and @ mention @gitlab-com/gl-security/appsec
• The MR includes necessary changes to maintain consistency between UI, API, email, or other methods
• Security reports checked/validated by a reviewer from the AppSec team