Revert "Fixes syntax error in older versions of graphviz."

This reverts commit 1368677 since
it introduced a lot regressions in pydot_ng when creating and
rendering dotfiles. It caused too much quotes to be added to
DOT representation of the graph which made graphs broken.

Author: prmtl

pydot_ng/__init__.py

@@ -243,10 +243,6 @@ def needs_quotes(s):
 
 
 def quote_if_necessary(s):
-    # Older versions of graphviz throws a syntax error for empty values without
-    # quotes, e.g. [label=]
-    if s == '':
-        return '""'
 
     if isinstance(s, bool):
         if s is True: