Empty classpath improvements

Previously, when there were no additional libraries, the classpath
would render as '-cp '.  This was a problem when that argument was
followed by something else.  Applications would end up with something
like '-cp  app.groovy' which was incorrect.  This change updates the
as_classpath() method to return nil if there is are no additional
libraries ensuring that the '-cp' argument is never added to the
command.

Author: nebhale

lib/java_buildpack/component/additional_libraries.rb

@@ -40,7 +40,7 @@ def initialize(droplet_root)
       def as_classpath
         qualified_paths = sort.map { |path| qualify_path path }
 
-        "-cp #{qualified_paths.join ':'}"
+        "-cp #{qualified_paths.join ':'}" unless empty?
       end
 
       # Symlink the contents of the collection to a destination directory.

spec/java_buildpack/component/additional_libraries_spec.rb

@@ -44,6 +44,11 @@
     end
   end
 
+  it 'should render empty string if classpath is empty' do
+    additional_libraries.clear
+    expect(additional_libraries.as_classpath).not_to be
+  end
+
   it 'should symbolically link additional libraries' do
     additional_libraries.link_to app_dir