Fix panic when a blockstring has only 1 line and blank

[miiton]

When I send a blockstring that has only 1 line and blank...

""""""

language/lexer.go panics.

panic: runtime error: index out of range [recovered]
        panic: runtime error: index out of range

This PR fix that.

[coveralls]

Coverage remained the same at 91.87% when pulling 63adf01 on miiton:fix-panic-blockstring into 21fe71d on graphql-go:master.

[chris-ramon]

Thanks a lot for fixing it @miiton 👍 , closing this one in favor of: #456