do not write empty or any markers

poetry/version/markers.py

@@ -628,7 +628,15 @@ def __hash__(self):
         return h
 
     def __str__(self):
-        return " or ".join(str(m) for m in self._markers)
+        return " or ".join(
+            str(m) for m in self._markers if not m.is_any() and not m.is_empty()
+        )
+
+    def is_any(self):
+        return all(m.is_any() for m in self._markers)
+
+    def is_empty(self):
+        return all(m.is_empty() for m in self._markers)
 
 
 def parse_marker(marker):

tests/version/test_markers.py

@@ -354,6 +354,32 @@ def test_marker_union_union_duplicates():
     )
 
 
+def test_marker_union_all_any():
+    union = MarkerUnion(parse_marker(""), parse_marker(""),)
+
+    assert union.is_any()
+
+
+def test_marker_union_all_any():
+    union = MarkerUnion(parse_marker("<empty>"), parse_marker("<empty>"),)
+
+    assert union.is_empty()
+
+
+def test_marker_str_conversion_skips_empty_and_any():
+    union = MarkerUnion(
+        parse_marker("<empty>"),
+        parse_marker(
+            'sys_platform == "darwin" or python_version <= "3.6" or os_name == "Windows"'
+        ),
+        parse_marker(""),
+    )
+
+    assert str(union) == (
+        'sys_platform == "darwin" or python_version <= "3.6" or os_name == "Windows"'
+    )
+
+
 def test_intersect_compacts_constraints():
     m = parse_marker('python_version < "4.0"')