Handle curses.tigetstr() returning None

Found with python/typeshed#6620

Author: JelleZijlstra

tornado/log.py

@@ -151,7 +151,11 @@ def __init__(
                     self._colors[levelno] = unicode_type(
                         curses.tparm(fg_color, code), "ascii"
                     )
-                self._normal = unicode_type(curses.tigetstr("sgr0"), "ascii")
+                normal = curses.tigetstr("sgr0")
+                if normal is not None:
+                    self._normal = unicode_type(normal, "ascii")
+                else:
+                    self._normal = ""
             else:
                 # If curses is not present (currently we'll only get here for
                 # colorama on windows), assume hard-coded ANSI color codes.