Made sure completions proc do not return nil

The completion proc returning nil causes errors in input systems that only
expect strings (e.g. Coolline).

lib/pry/completion.rb

@@ -260,7 +260,7 @@ class Pry
     end
 
     def self.select_message(path, receiver, message, candidates)
-      candidates.grep(/^#{message}/).collect do |e|
+      candidates.grep(/^#{message}/).collect { |e|
         case e
         when /^[a-zA-Z_]/
           path.call(receiver + "." + e)
@@ -268,7 +268,7 @@ class Pry
         when *Operators
           #receiver + " " + e
         end
-      end
+      }.compact
     end
 
     # build_path seperates the input into two parts: path and input.
@@ -293,4 +293,3 @@ class Pry
     end
   end
 end
-

spec/completion_spec.rb

@@ -226,4 +226,8 @@ describe Pry::InputCompleter do
     completer_test(b, pry).call('/Con')
   end
 
+  it 'should not return nil in its output' do
+    pry = Pry.new
+    new_completer(binding, pry).call("pry.").should.not.include nil
+  end
 end