Check null to avoid null pointer exception

[bact]

Check if the string input is null in getFirstLicenseToken and isSingleTokenString

To fix #333

[goneall]

Since we're now using a Java version that support the @Nullable annotations, we should add these to the calls.

Do we want to open a separate PR to fix the empty optional strings returning true when it should be false?

[bact]

Do we want to open a separate PR to fix the empty optional strings returning true when it should be false?

We can fix it here as well.
I also have to update the unit test to expect false when the string is empty.