Unable to use stream maps for string field.

Summary

Steps to reproduce

I have the following added to my config: "stream_maps": { "survey_responses": { "email_address": "'string_to_replace'" } The top level property in the schema looks like: "email_address": { "type": "string" }

What is the current bug behavior?

The Property that the mapper creates will always be optional When the to_dict() gets called on the property, append_type will always fail here.

What is the expected correct behavior?

append_type shouldn't get called in this case. It works if commented out.

Relevant logs and/or screenshots

Traceback (most recent call last):
  File "<string>", line 1, in <module>
  File "/Users/dan.ladd/VSCode/tap-surveymonkey/.venv/lib/python3.8/site-packages/click/core.py", line 829, in __call__
    return self.main(*args, **kwargs)
  File "/Users/dan.ladd/VSCode/tap-surveymonkey/.venv/lib/python3.8/site-packages/click/core.py", line 782, in main
    rv = self.invoke(ctx)
  File "/Users/dan.ladd/VSCode/tap-surveymonkey/.venv/lib/python3.8/site-packages/click/core.py", line 1066, in invoke
    return ctx.invoke(self.callback, **ctx.params)
  File "/Users/dan.ladd/VSCode/tap-surveymonkey/.venv/lib/python3.8/site-packages/click/core.py", line 610, in invoke
    return callback(*args, **kwargs)
  File "/Users/dan.ladd/VSCode/tap-surveymonkey/.venv/lib/python3.8/site-packages/singer_sdk/tap_base.py", line 319, in cli
    tap = cls(  # type: ignore  # Ignore 'type not callable'
  File "/Users/dan.ladd/VSCode/tap-surveymonkey/.venv/lib/python3.8/site-packages/singer_sdk/tap_base.py", line 62, in __init__
    self.mapper.register_raw_streams_from_catalog(
  File "/Users/dan.ladd/VSCode/tap-surveymonkey/.venv/lib/python3.8/site-packages/singer_sdk/mapper.py", line 364, in register_raw_streams_from_catalog
    self.register_raw_stream_schema(
  File "/Users/dan.ladd/VSCode/tap-surveymonkey/.venv/lib/python3.8/site-packages/singer_sdk/mapper.py", line 427, in register_raw_stream_schema
    mapper = CustomStreamMap(
  File "/Users/dan.ladd/VSCode/tap-surveymonkey/.venv/lib/python3.8/site-packages/singer_sdk/mapper.py", line 120, in __init__
    ) = self._init_functions_and_schema(stream_map=map_transform)
  File "/Users/dan.ladd/VSCode/tap-surveymonkey/.venv/lib/python3.8/site-packages/singer_sdk/mapper.py", line 239, in _init_functions_and_schema
    Property(
  File "/Users/dan.ladd/VSCode/tap-surveymonkey/.venv/lib/python3.8/site-packages/singer_sdk/typing.py", line 175, in to_dict
    type_dict = append_type(type_dict, "null")
  File "/Users/dan.ladd/VSCode/tap-surveymonkey/.venv/lib/python3.8/site-packages/singer_sdk/helpers/_typing.py", line 46, in append_type
    raise ValueError("Could not append type because type was not detected.")
ValueError: Could not append type because type was not detected.

Possible fixes