License exception fix when info is not available

Fellow Hampei,

I’ve fixed an exception in license parse logic which happens when information on AppLicense (that is optional and not always available) makes your gem useless. 

According to the information on RM Unify Dashboard for SSO, the following parameters are optional and not always available on callback page: 

AppLicence  	http://schemas.rm.com/identity/claims/applicence
AppEstablishmentKey  	http://schemas.rm.com/identity/claims/appestablishmentkey
AppUserId  	http://schemas.rm.com/identity/claims/appuserid

As a result of this, Rails raises exception on line 43 of your code:
licence = Hash[licence_str[1..-2].split('|').map{|kv|

This commit fixes it.

If you need more information or full log that contains exception, feel free to drop me a line.

P.S. This code is probably not 100% perfect and I assume that there would be a more elegant solution. I'm relatively new to Ruby On Rails and study it ATM.

Thanks!
Variable will always be defined, since you assign to it on line one. Will be nil if key is missing from hash. 

I would just add this as the second line: 

return nil if licence_str.nil? || licence_str.empty?

And just make that the only thing this pull-request changes.