Fix use of OPENSSL in SSL tests if command is not found

`openssl` is an optional dependency in the meson build as it may not be
installed in an environment even if SSL libraries are around.  The meson
scripts assume that, but the SSL tests thought that it was a hard
dependency, causing a meson installation to fail if `openssl` could not
be found.  Like similar tests that depend on external commands, and to
be consistent with ./configure for the SSL tests, this commit makes the
command existence optional in the tests.

Author: Tristan Partin
Discussion: https://postgr.es/m/CWSX6P5OUUM5.N7B74KQ06ZP6@neon.tech
Backpatch-through: 16

src/test/ssl/meson.build

@@ -7,7 +7,7 @@ tests += {
   'tap': {
     'env': {
       'with_ssl': ssl_library,
-      'OPENSSL': openssl.path(),
+      'OPENSSL': openssl.found() ? openssl.path() : '',
     },
     'tests': [
       't/001_ssltests.pl',