Fix stray stderr output on machines with GNU which

On a NixOS machine I was getting a message like the following on every
Rails boot:

> which: no bun in (... [my path] ...)

I chased it down to this library's usage of `which` to find the mjml
binary. Using backticks to call it meant stderr was passing through to
the terminal.

Ubuntu and MacOS use different versions of `which` that do not
print to stderr, so it's likely not many people have run into this
issue.

lib/mjml.rb

@@ -74,8 +74,9 @@ def self.check_for_custom_mjml_binary
   end
 
   def self.check_for_mjml_package(package_manager, bin_command, binary_path)
-    pm_bin = `which #{package_manager}`.chomp
-    return if pm_bin.blank?
+    stdout, _, status = Open3.capture3("which #{package_manager}")
+    return unless status.success?
+    pm_bin = stdout.chomp
 
     stdout, _, status = Open3.capture3("#{pm_bin} #{bin_command}")
     return unless status.success?
@@ -93,11 +94,14 @@ def self.check_for_package_mjml_binary
   end
 
   def self.check_for_global_mjml_binary
-    mjml_bin = `which mjml`.chomp
+    stdout, _, status = Open3.capture3("which mjml")
+    return unless status.success?
+    mjml_bin = stdout.chomp
     return mjml_bin if mjml_bin.present? && check_version(mjml_bin)
   end
 
   def self.check_for_mrml_binary
+    return unless Mjml.use_mrml
     MRML.present?
   rescue NameError
     Mjml.mjml_binary_error_string = 'Couldn\'t find MRML - did you add \'mrml\' to your Gemfile?'

test/fixtures/valid-mjml-binary/empty-path/which

@@ -0,0 +1,7 @@
+#!/bin/sh
+
+# simulate GNU which's stderr message when it cannot find something
+>&2 echo "I should not show up in test output"
+
+# simulate an empty path by always returning nonzero
+exit 1

test/mjml_test.rb

@@ -168,8 +168,19 @@ class NotifierMailerTest < ActiveSupport::TestCase
 
       Mjml.valid_mjml_binary = nil
       MRML.stubs(:present?).raises(NameError)
+      Mjml.stubs(:puts) # silence printed error message from test output
       assert_nil(Mjml.valid_mjml_binary)
       expect(Mjml.mjml_binary_error_string).must_equal 'Couldn\'t find MRML - did you add \'mrml\' to your Gemfile?'
     end
+
+    it "with nothing on the path, sets valid_mjml_binary to nil" do
+      old_path = ENV["PATH"]
+      ENV["PATH"] = "./test/fixtures/valid-mjml-binary/empty-path/"
+
+      Mjml.stubs(:puts) # silence printed error message from test output
+      assert_nil(Mjml.valid_mjml_binary)
+    ensure
+      ENV["PATH"] = old_path
+    end
   end
 end