Fix conversion table generator scripts.

Fix conversion table generator scripts.

convutils.pm used implicit conversion of undefined value to integer
zero.  Some of conversion scripts are susceptible to regexp greediness.
Fix, avoiding whitespace changes in the output.  Also update ICU URLs
that moved.

No need to back-patch, because the output of these scripts is also in
the source tree so we shouldn't need to rerun them on back-branches.

Author: Kyotaro Horiguchi <horikyoga.ntt@gmail.com>
Discussion: https://postgr.es/m/CA%2BhUKGJ7SEGLbj%3D%3DTQCcyKRA9aqj8%2B6L%3DexSq1y25TA%3DWxLziQ%40mail.gmail.com