Symbols: Set symbol_shift default rather than None

Since all the checks for symbol_shift use essentially "if not
config['symbol_shift']" is doesn't matter whether 0 or None is
returned.  I'd like to test this on an ASLR image, but I think it should
be fine and I'd feel much happier about everything if we could give it a
numeric default.

volatility3/framework/interfaces/symbols.py

@@ -303,7 +303,8 @@ class SymbolTableInterface(BaseSymbolTableInterface, configuration.ConfigurableI
     @classmethod
     def get_requirements(cls) -> List[RequirementInterface]:
         return super().get_requirements() + [
-            requirements.IntRequirement(name = 'symbol_shift', description = 'Symbol Shift', optional = True),
+            requirements.IntRequirement(
+                name = 'symbol_shift', description = 'Symbol Shift', optional = True, default = 0),
             requirements.IntRequirement(
                 name = 'symbol_mask', description = 'Address mask for symbols', optional = True, default = 0),
         ]