fix concat operator with undef variables

[kgoess]

Generally xslate handles undefined $variables silently. But in the case of concatenation with the ~ operator, it dies with the message

Use of uninitialized value in subroutine entry

This change to the concat operator handling makes $A ~ $B not fail if either is undef, which is consistent with the rest of xslate's behavior.

[syohex]

https://github.com/xslate/p5-Text-Xslate/pull/169/commits/5f0c854e6f2b62ced4bd266129313ba56569a415

You need not to add PP test. All tests are checked with both XS and PP version by switching backend via environment variable.

[syohex]

Could you squash commits ?

[kgoess]

Commits are now squashed!

[syohex]

Thanks