FindRuby: Fix output check in _RUBY_CONFIG_VAR

Since commit v2.8.8~173^2 (FindRuby: clean up querying variables from
Ruby, 2012-02-17) we query RbConfig::CONFIG first and, if the command
fails or its output equates to a false constant, then fall back to
querying Config::CONFIG.

Due to the above, an error condition exists with Ruby 2.2.0; when
querying RbConfig::CONFIG['TEENY'], the output of '0' will be discarded
since it matches the false constant '0'.

In previous versions this wasn't a problem, but Ruby 2.2 has completely
removed Config::CONFIG. This causes RUBY_VERSION_PATCH to be set to an
empty string and the Ruby version to be detected as '2.2.' (instead of
'2.2.0').

Fix the output check to explicitly look for an empty string before using
the fallback query method. (Someone more familiar with Ruby might be
able to deem the fallback as unnecessary and fully remove it.)

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