[lit] Use canonical None test

[ychen]

Summary

According to here
https://stackoverflow.com/questions/3965104/not-none-test-in-python

[merge_guards_bot]

Unit tests: fail. 62154 tests passed, 5 failed and 812 were skipped.

failed: libc++.std/language_support/cmp/cmp_partialord/partialord.pass.cpp
failed: libc++.std/language_support/cmp/cmp_strongeq/cmp.strongeq.pass.cpp
failed: libc++.std/language_support/cmp/cmp_strongord/strongord.pass.cpp
failed: libc++.std/language_support/cmp/cmp_weakeq/cmp.weakeq.pass.cpp
failed: libc++.std/language_support/cmp/cmp_weakord/weakord.pass.cpp

clang-tidy: pass.

clang-format: pass.

Build artifacts: diff.json, clang-tidy.txt, clang-format.patch, CMakeCache.txt, console-log.txt, test-results.xml

Pre-merge checks is in beta. Report issue. Please join beta or enable it for your project.

[merge_guards_bot]

Unit tests: pass. 62278 tests passed, 0 failed and 827 were skipped.

clang-tidy: pass.

clang-format: pass.

Build artifacts: diff.json, clang-tidy.txt, clang-format.patch, CMakeCache.txt, console-log.txt, test-results.xml

Pre-merge checks is in beta. Report issue. Please join beta or enable it for your project.

llvm/utils/lit/lit/TestRunner.py

for i,j in enumerate(cmd.commands):
 
         # Resolve the executable path ourselves.
         executable = None
         # For paths relative to cwd, use the cwd of the shell environment.
         if args[0].startswith('.'):
             exe_in_cwd = os.path.join(cmd_shenv.cwd, args[0])
             if os.path.isfile(exe_in_cwd):
                 executable = exe_in_cwd
-        if not executable:
+        if executable is None:
             executable = lit.util.which(args[0], cmd_shenv.env['PATH'])
-        if not executable:
+        if executable is None:
             raise InternalShellError(j, '%r: command not found' % args[0])
 
         # Replace uses of /dev/null with temporary files.
         if kAvoidDevNull:
             # In Python 2.x, basestring is the base class for all string (including unicode)
             # In Python 3.x, basestring no longer exist and str is always unicode
             try:
                 str_type = basestring