unaccent: Tweak value of PYTHON when building without Python support

unaccent: Tweak value of PYTHON when building without Python support

As coded, the module's Makefile would fail to set a value for PYTHON as
it checked if the variable is defined.  When compiling without
--with-python, PYTHON is defined and set to an empty value, so the
existing check is not able to do its work.

This commit switches the rule to check if the value is empty rather than
defined, allowing the generation of unaccent.rules even if --with-python
is not used as long as "python" exists.  BISON and FLEX do the same in
pgxs.mk, for instance.

Thinko in f85a485f89e2.

Author: Japin Li
Discussion: https://postgr.es/m/MEYP282MB1669F86C0DC7B4DC48489CB0B6C3A@MEYP282MB1669.AUSP282.PROD.OUTLOOK.COM
Backpatch-through: 13