fix: add error check in jmespath type conversion in context variables

[vishal-chdhry]

Explanation

The following policy and pod caused panic in admission and reports-controller

apiVersion: kyverno.io/v1
kind: ClusterPolicy
metadata:
  name: test-panic
spec:
  validationFailureAction: Enforce 
  background: true 
  rules:
    - name: test-panic
      match:
        any:
        - resources:
            kinds:
              - Pod
      context:
      - name: image
        variable:
          jmesPath: '{{ request.object.spec.[containers,initContainers, ephemeralContainers][].image[] }}'
      validate:
        deny:
          conditions:
            all:
            - key: "{{ image }}"
              operator: AnyNotIn
              value: 
              - "gcr.io/datadoghq/agent" 

Apply the following pod

$ kubectl run test --image nginx     
Error from server (InternalError): Internal error occurred: failed calling webhook "validate.kyverno.svc-fail": failed to call webhook: Post "[https://kyverno-svc.kyverno.svc:443/validate/fail?timeout=10s](https://kyverno-svc.kyverno.svc/validate/fail?timeout=10s)": dial tcp 10.96.166.0:443: connect: connection refused

This is because the path variable resolves to empty value and its type conversion caused a panic, this PR makes that type conversion safer.

Related issue

NA

Milestone of this PR

Documentation (required for features)

My PR contains new or altered behavior to Kyverno.

• I have sent the draft PR to add or update the documentation and the link is:

What type of PR is this

Proposed Changes

Proof Manifests

Checklist

• I have read the contributing guidelines.
• I have read the PR documentation guide and followed the process including adding proof manifests to this PR.
• This is a bug fix and I have added unit tests that prove my fix is effective.
• This is a feature and I have added CLI tests that are applicable.
• My PR needs to be cherry picked to a specific release branch which is .
• My PR contains new or altered behavior to Kyverno and
  • CLI support should be added and my PR doesn't contain that functionality.

Further Comments

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 10.15%. Comparing base (5d50022) to head (1584589).

Files	Patch %	Lines
pkg/engine/context/loaders/variable.go	0.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main   #10152      +/-   ##
==========================================
- Coverage   10.15%   10.15%   -0.01%     
==========================================
  Files        1030     1030              
  Lines       91779    91782       +3     
==========================================
  Hits         9323     9323              
- Misses      81438    81441       +3     
  Partials     1018     1018              

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[realshuting]

/cherry-pick release-1.12