Handle missing or invalid contexts in "not-in-future" sev validations.

[stevenbronson-wk]

We encountered an error running EDGAR validation on a document with an invalid context:

Traceback (most recent call last):
  File "[...]/arelle/Validate.py", line 131, in validate
    self.instValidator.validate(self.modelXbrl, self.modelXbrl.modelManager.formulaOptions.typedParameters(self.modelXbrl.prefixedNamespaces))
    ~~~~~~~~~~~~~~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "[...]/arelle/ValidateXbrl.py", line 399, in validate
    pluginXbrlMethod(self)
    ~~~~~~~~~~~~~~~~^^^^^^
  File "[...]/edgar/validate/__init__.py", line 408, in validateXbrlFinally
    validateFiling(val, modelXbrl, isEFM=True)
    ~~~~~~~~~~~~~~^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "[...]/edgar/validate/Filing.py", line 2097, in validateFiling
    if deiDocumentType and f.context.endDatetime > documentTypeFact.context.endDatetime:
                           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
TypeError: '>' not supported between instances of 'NoneType' and 'datetime.datetime'

cc @freddym56