polar: allow properly specifying polar_dir, with ~ as a default

tests/extra/polar.py

@@ -7,18 +7,21 @@ import pytest # type: ignore
 
 # todo maybe search fot info.json recursively?
 @pytest.mark.parametrize('dotpolar', [
+    '',
     'data/polar/BojanKV_polar/.polar',
     'data/polar/TheCedarPrince_KnowledgeRepository',
     'data/polar/coelias_polardocs',
     'data/polar/warkdarrior_polar-document-repository'
 ])
 def test_hpi(dotpolar: str):
-    pdir = Path(ROOT / dotpolar)
-    class user_config:
-        export_dir = pdir
+    if dotpolar != '':
+        pdir = Path(ROOT / dotpolar)
+        class user_config:
+            export_dir = pdir
+
+        import my.config
+        setattr(my.config, 'polar', user_config)
 
-    import my.config
-    setattr(my.config, 'polar', user_config)
     import sys
     M = 'my.reading.polar'
     if M in sys.modules: