Unoptimize 'read' to return freshly allocated empty strings

Unoptimize 'read' to return freshly allocated empty strings

* libguile/read.c (scm_read_string): Return a freshly allocated string
  every time, even for empty strings.  The motivation is to allow source
  properties to be added to all strings.  Previously, the shared global
  'scm_nullstr' was returned for empty strings.  Note that empty strings
  still share a common global 'null_stringbuf'.

* test-suite/tests/srfi-13.test (substring/shared): Fix tests to reflect
  the fact that empty string literals are no longer guaranteed to be
  'eq?' to each other.