Fix a large number of typing issues.

There are several instances where mypy didn't detect

if blah is not None:
    blah = thing

and so were rewritten as:

blah = blah or thing

volatility/framework/interfaces/objects.py

@@ -1,13 +1,12 @@
 """Objects are the core of volatility, and provide pythonic access to interpreted values of data from a layer.
 """
 
+import collections
+import collections.abc
 import logging
 import typing
 from abc import ABCMeta, abstractmethod
 
-import collections
-import collections.abc
-
 from volatility.framework import constants, validity, interfaces
 from volatility.framework.interfaces import context as interfaces_context
 
@@ -109,7 +108,7 @@ class ObjectInterface(validity.ValidityRoutines, metaclass = ABCMeta):
         Raises InvalidDataException on failure to validate the data correctly.
         """
 
-    def get_symbol_table(self) -> 'interfaces.symbols.SymbolTableInterface':
+    def get_symbol_table(self) -> typing.Optional['interfaces.symbols.SymbolTableInterface']:
         """Returns the symbol table for this particular object
 
         Returns none if the symbol table cannot be identified.