Make target_uri default to "/" in case the TARGETURI option is nil or empty

lib/msf/core/exploit/http/client.rb

@@ -524,7 +524,10 @@ module Exploit::Remote::HttpClient
 	#
 	def target_uri
 		begin
-			URI(datastore['TARGETURI'])
+			# In case TARGETURI is empty, at least we default to '/'
+			u = datastore['TARGETURI']
+			u = "/" if u.nil? or u.empty?
+			URI(u)
 		rescue ::URI::InvalidURIError
 			print_error "Invalid URI: #{datastore['TARGETURI'].inspect}"
 			raise Msf::OptionValidateError.new(['TARGETURI'])