RFR: 8215401: Add isEmpty default method to CharSequence

[Claes Redestad]

Hi,

let's add an isEmpty default method to CharSequence!

Webrev: http://cr.openjdk.java.net/~redestad/8215401/open.00/
RFE:    https://bugs.openjdk.java.net/browse/JDK-8215401
CSR:    https://bugs.openjdk.java.net/browse/JDK-8215402

Testing: tier1-3

/Claes