Fix TypeError in report_collect with _collect_report_last_write

`_collect_report_last_write` might be None, when `pytest_collection` was
not called before.  Not sure if this indicates another problem, but it
can be reproduced with `testing/test_collection.py::TestCollector::()::test_getcustomfile_roundtrip`.

Fixes pytest-dev#4329

changelog/4329.bugfix.rst

@@ -0,0 +1 @@
+Fix TypeError in report_collect with _collect_report_last_write.

src/_pytest/terminal.py

@@ -497,7 +497,10 @@ def report_collect(self, final=False):
         if not final:
             # Only write "collecting" report every 0.5s.
             t = time.time()
-            if self._collect_report_last_write > t - 0.5:
+            if (
+                self._collect_report_last_write is not None
+                and self._collect_report_last_write > t - 0.5
+            ):
                 return
             self._collect_report_last_write = t