azurerm_policy_set_definition: fix acceptance test policy set with no parameter

[wuxu92]

We should set Parameters only when specified in configuration.

current test will fail with:

=== CONT  TestAccAzureRMPolicySetDefinition_customNoParameter
    testcase.go:113: Step 1/2 error: After applying this test step, the plan was not empty.
        stdout:
        Terraform used the selected providers to generate the following execution
        plan. Resource actions are indicated with the following symbols:
          ~ update in-place
        Terraform will perform the following actions:
          # azurerm_policy_set_definition.test will be updated in-place
          ~ resource "azurerm_policy_set_definition" "test" {
                id           = "/subscriptions/*******/providers/Microsoft.Authorization/policySetDefinitions/acctestPolSet-230901045501469094"
                name         = "acctestPolSet-230901045501469094"
                # (3 unchanged attributes hidden)
              ~ policy_definition_reference {
                  - parameter_values     = jsonencode({})
                  + policy_group_names   = []
                    # (2 unchanged attributes hidden)
                }
            }
        Plan: 0 to add, 1 to change, 0 to destroy.
--- FAIL: TestAccAzureRMPolicySetDefinition_customNoParameter (218.84s)

Test Pass now:

=== RUN   TestAccAzureRMPolicySetDefinition_customNoParameter
=== PAUSE TestAccAzureRMPolicySetDefinition_customNoParameter
=== CONT  TestAccAzureRMPolicySetDefinition_customNoParameter
--- PASS: TestAccAzureRMPolicySetDefinition_customNoParameter (243.94s)

[tombuildsstuff]

LGTM 👍