ARC/newruntime: strutils.join on seq with only empty strings causes segfault

[zedeus]

Possibly related to #12961, but when not using newruntime there's no IndexError.

import strutils
echo @[""].join()
echo @["", ""].join()

Traceback (most recent call last)
/tmp/bug.nim(17)         bug
/home/zed/.choosenim/toolchains/nim-#devel/lib/pure/strutils.nim(1765) join
SIGSEGV: Illegal storage access. (Attempt to read from nil?)
Error: execution of an external program failed: '/tmp/bug '