Optimized away the creation of empty string objects.

[haberman]

Prior to this CL, creating an empty message object would create
two empty string objects for every declared field. First we
created a unique string object for the field's default. Then
we created yet another string object when we assigned the
default value into the message: we called #encode to ensure
that the string would have the correct encoding and be frozen.

I optimized these unnecessary objects away with two fixes:

1. Memoize the empty string so that we don't create a new empty
   string for every field's default.
2. If we are assigning a string to a message object, avoid creating
   a new string if the assigned string has the correct encoding and
   is already frozen.

[haberman]

This PR fixes #6496.