idnatest error if xn-- empty or all-ASCII

[markusicu]

[165-A48] Action Item for Markus Scherer, Editorial Committee: Update UTS#46 to validate ACE label edge cases, see L2/20-240 item F7. For Unicode 14.

Corresponds to spec change

• https://github.com/unicode-org/unicode-reports/pull/149

L2/20-240 item F7

The IDNA2008 ToUnicode operation validates ACE labels ("xn--" plus Punycode)
by decoding them, then re-encoding via ToASCII, and verifying that the
round-trip output is the same as the input (case-insensitive).

The UTS#46 ToUnicode operation and its Processing step uses a cheaper
Convert/Validate step which wants to be equivalent.

However, it misses two edge cases which pass Convert/Validate step but which
IDNA2008 catches with its round-trip verification:

1. "xn--" decodes to an empty string
2. "xn--ASCII-" decodes to just "ASCII"

I propose that we modify
https://www.unicode.org/reports/tr46/#ProcessingStepPunycode (section 4
Processing > step 4 "Convert/Validate" > If the label starts with
“xn--”) so that it catches these cases.

...