Fix error when there are empty lines in real certificate file

[imnuke]

When there are empty lines in the fullchail certificate file in real_certs, empty lines are incorrectly interpreted as certificates. This causes OpenSSL.crypto.load_certificate to throw an error.

acme.sh, a widely-used tool for certificate management, produces certificate chains in a format with empty lines separating individual certificates:

-----BEGIN CERTIFICATE-----
...
-----END CERTIFICATE-----

-----BEGIN CERTIFICATE-----
...
-----END CERTIFICATE-----

Traceback (most recent call last):
  File ".../certmitm/certmitm.py", line 55, in threaded_connection_handler
    test = connection_tests.get_test(connection)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".../certmitm/certmitm/connection.py", line 72, in get_test
    next_test = self.all_test_dict[connection.identifier].get_test()
                ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".../certmitm/certmitm/connection.py", line 123, in get_test
    for test in certmitm.certtest.generate_test_context(self.upstream_cert_fullchain, self.connection.upstream_sni or self.connection.upstream_ip, self.working_dir, self.logger):
  File ".../certmitm/certmitm/certtest.py", line 87, in generate_test_context
    cert = OpenSSL.crypto.load_certificate(OpenSSL.crypto.FILETYPE_PEM, real_cert_pem)
           ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
  File ".../certmitm/lib/python3.12/site-packages/OpenSSL/crypto.py", line 2090, in load_certificate
    _raise_current_error()
  File ".../.pyenv/versions/certmitm/lib/python3.12/site-packages/OpenSSL/_util.py", line 57, in exception_from_error_queue
    raise exception_type(errors)

This fix modifies the certificate processing to skip empty lines, ensuring that only valid certificates are passed to OpenSSL.crypto.load_certificate. This change enhances compatibility with acme.sh output and prevents unnecessary errors.