api/benchmark-results: supplying earliest_timestamp=NA results in a 500

[austin3dickey]

See, e.g., https://conbench.ursa.dev/api/benchmark-results/?earliest_timestamp=NA :

unexpected exception, please report this:
Traceback (most recent call last):
  File "/usr/local/lib/python3.11/site-packages/flask/app.py", line 1823, in full_dispatch_request
    rv = self.dispatch_request()
         ^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/flask/app.py", line 1799, in dispatch_request
    return self.ensure_sync(self.view_functions[rule.endpoint])(**view_args)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/flask/views.py", line 107, in view
    return current_app.ensure_sync(self.dispatch_request)(**kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/usr/local/lib/python3.11/site-packages/flask/views.py", line 188, in dispatch_request
    return current_app.ensure_sync(meth)(**kwargs)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "/app/conbench/api/_endpoint.py", line 23, in maybe
    return func(*args, **kwargs)
           ^^^^^^^^^^^^^^^^^^^^^
  File "/app/conbench/api/results.py", line 224, in get
    pd.Timestamp(earliest_timestamp_arg).timestamp() * 10**9
    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File "timestamps.pyx", line 1899, in pandas._libs.tslibs.timestamps.Timestamp.__new__
  File "conversion.pyx", line 269, in pandas._libs.tslibs.conversion.convert_to_tsobject
  File "conversion.pyx", line 546, in pandas._libs.tslibs.conversion.convert_str_to_tsobject
  File "parsing.pyx", line 331, in pandas._libs.tslibs.parsing.parse_datetime_string
  File "parsing.pyx", line 660, in pandas._libs.tslibs.parsing.dateutil_parse
pandas._libs.tslibs.parsing.DateParseError: Unknown datetime string format, unable to parse: NA

This should be an expected 400, not a 500.