fix(vpn_gateway_connection): adding support to null patch ike_policy and ipsec_policy on vpn_gateway_connection

[ujjwal-ibm]

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Relates OR Closes #0000

Output from acceptance testing:

--- PASS: TestAccIBMISVPNGatewayConnection_ike_ipsec_null_patch (544.89s)
PASS
ok      github.com/IBM-Cloud/terraform-provider-ibm/ibm/service/vpc     546.422s

[ujjwal-ibm]

--- PASS: TestAccIBMISVPNGatewayConnection_ike_ipsec_null_patch (626.13s)
PASS
ok      github.com/IBM-Cloud/terraform-provider-ibm/ibm/service/vpc     629.663s