brew --config: properly handle empty which strings

Symptom: If no python/ruby/perl is in your path, then `which x`.chomp
returns an empty string and `unless ""` is still true.  So, N/A is never
displayed.  Instead, ruby's Pathname.new("").realpath returns the cwd.
(I consider this realpath behavior a ruby bug) Fix: use empty?

Closes Homebrew/homebrew#10027.

Signed-off-by: Jack Nagel <jacknagel@gmail.com>

Library/Homebrew/cmd/--config.rb

@@ -38,7 +38,7 @@ module Homebrew extend self
 
   def describe_perl
     perl = `which perl`.chomp
-    return "N/A" unless perl
+    return "N/A" if perl.empty?
 
     real_perl = Pathname.new(perl).realpath.to_s
     return perl if perl == real_perl
@@ -47,7 +47,7 @@ module Homebrew extend self
 
   def describe_python
     python = `which python`.chomp
-    return "N/A" unless python
+    return "N/A" if python.empty?
 
     real_python = Pathname.new(python).realpath.to_s
 
@@ -57,7 +57,7 @@ module Homebrew extend self
 
   def describe_ruby
     ruby = `which ruby`.chomp
-    return "N/A" unless ruby
+    return "N/A" if ruby.empty?
 
     real_ruby = Pathname.new(ruby).realpath.to_s
     return ruby if ruby == real_ruby