Fix empty hash console rendering bug

Previously, if a face action returned an empty hash Puppet would print
out an error something like the following:

    err: undefined method `+' for nil:NilClass
    err: Try 'puppet help stack build' for usage

The cause of the error was a method in network/formats.rb that did not
account for the possibilty of an empty hash. This commit resolves the
issue by adding logic to account for the empty hash scenario, rendering
an empty hash as an empty string.

lib/puppet/network/formats.rb

@@ -148,7 +148,7 @@ def render(datum)
     # Simple hash to table
     if datum.is_a? Hash and datum.keys.all? { |x| x.is_a? String or x.is_a? Numeric }
       output = ''
-      column_a = datum.map do |k,v| k.to_s.length end.max + 2
+      column_a = datum.empty? ? 2 : datum.map{ |k,v| k.to_s.length }.max + 2
       column_b = 79 - column_a
       datum.sort_by { |k,v| k.to_s } .each do |key, value|
         output << key.to_s.ljust(column_a)

spec/unit/network/formats_spec.rb

@@ -311,6 +311,10 @@ def to_pson(*args)
       end
     end
 
+    it "should render empty hashes as empty strings" do
+      subject.render({}).should == ''
+    end
+
     it "should render a non-trivially-keyed Hash as JSON" do
       hash = { [1,2] => 3, [2,3] => 5, [3,4] => 7 }
       subject.render(hash).should == json.render(hash).chomp