converting empty rfc5280.SubjectAltName to bool raises PyAsn1Error

[oconnor663]

Here's an example script demonstrating the crash:

import pyasn1_modules
from pyasn1_modules import rfc5280

print(pyasn1_modules.__version__)

nonempty_san = rfc5280.SubjectAltName()
nonempty_san.append(rfc5280.GeneralName())
# This works fine.
bool(nonempty_san)

empty_san = rfc5280.SubjectAltName()
# This crashes.
bool(empty_san)

$ python test.py
0.3.0
Traceback (most recent call last):
  File "/tmp/test.py", line 13, in <module>
    bool(empty_san)
  File "/tmp/venv.QYC/lib/python3.9/site-packages/pyasn1/type/base.py", line 571, in __bool__
    return bool(self.components)
  File "/tmp/venv.QYC/lib/python3.9/site-packages/pyasn1/type/univ.py", line 1944, in components
    for idx in sorted(self._componentValues)]
  File "/tmp/venv.QYC/lib/python3.9/site-packages/pyasn1/type/base.py", line 213, in plug
    raise error.PyAsn1Error('Attempted "%s" operation on ASN.1 schema object' % name)
pyasn1.error.PyAsn1Error: Attempted "__iter__" operation on ASN.1 schema object