FIX: do not raise error if configuration file is missing

Summary

see https://gitlab.com/linkahead/linkahead-pylib/-/issues/107

Focus

The reason, why this worked before, is that the ConfigParser was given a non existing file to read from. The parser did not complain but created an empty config. We now check whether the file exists before calling configure, thus we must make sure, that we at least have an empty config.

Test Environment

How to set up a test environment for manual testing?

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unittests/test_configuration.py

@@ -22,10 +22,11 @@
 # ** end header
 #
 
-import pytest
-import linkahead as db
 from os import environ, getcwd, remove
 from os.path import expanduser, isfile, join
+
+import linkahead as db
+import pytest
 from pytest import raises
 
 
@@ -55,7 +56,8 @@ def test_config_ini_via_envvar(temp_ini_files):
     environ["PYCAOSDBINI"] = "bla bla"
     assert environ["PYCAOSDBINI"] == "bla bla"
     # test wrong configuration file in envvar
-    assert not expanduser(environ["PYCAOSDBINI"]) in db.configuration._read_config_files()
+    with pytest.raises(RuntimeError):
+        db.configuration._read_config_files()
     # test good configuration file in envvar
     environ["PYCAOSDBINI"] = "~/.pylinkahead.ini"
     assert expanduser("~/.pylinkahead.ini") in db.configuration._read_config_files()