Fixed possible Nullpointexception when checking subscription status in Java

[Jethro87]

Motivation / Description

We now check whether the entitlement exists before accessing .isActive().

Old

if (customerInfo.getEntitlements().get(<your_entitlement_id>).isActive()) {
	// user has access to "your_entitlement_id"
}

New

if (customerInfo.getEntitlements().get(<your_entitlement_id>) != null
    && customerInfo.getEntitlements().get(<your_entitlement_id>).isActive()) {
	// user has access to "your_entitlement_id"
}

[github-actions]

Preview this PR here: https://dev-docs.revenuecat.com/pr-608/