bpo-34897: avoid distutils test error when CXX is not set (GH-9706)

Depending on system config, a missing candidate compiler name may be
reported as the empty string rather than as None, so adjust the test
helper accordingly.

Lib/test/support/__init__.py

@@ -2783,7 +2783,7 @@ def missing_compiler_executable(cmd_names=[]):
         if cmd_names:
             assert cmd is not None, \
                     "the '%s' executable is not configured" % name
-        elif cmd is None:
+        elif not cmd:
             continue
         if spawn.find_executable(cmd[0]) is None:
             return cmd[0]

Misc/NEWS.d/next/Library/2018-10-04-20-25-35.bpo-34897.rNE2Cy.rst

@@ -0,0 +1,2 @@
+Adjust test.support.missing_compiler_executable check so that a nominal
+command name of "" is ignored. Patch by Michael Felt.