Check install_requires/extras_require for nullable

On Windows install_requires/extras_require might be None
if there is nothing to install.

See:
https://ci.appveyor.com/project/jazzband/pip-tools/builds/26819811/job/33bc9bb11e14skvv
https://ci.appveyor.com/project/jazzband/pip-tools/builds/26819708/job/igo2ih808la8fum6

piptools/scripts/compile.py

@@ -302,19 +302,21 @@ def cli(
                 from distutils.core import run_setup
 
                 dist = run_setup(src_file)
-                tmpfile.write("\n".join(dist.install_requires))
+                if dist.install_requires:
+                    tmpfile.write("\n".join(dist.install_requires))
 
                 # Parse extras_require of the form
                 # "extra:{marker}": ["package"] to
                 # "package ; {marker}" if extra is empty.
-                for extra_and_marker, packages in dist.extras_require.items():
-                    extra, marker = extra_and_marker.split(":")
-                    # TODO: non-empty extras will be ignored, perhaps we need
-                    #  an option like --with-extra to specify one. See GH-625.
-                    if extra:
-                        continue
-                    for package in packages:
-                        tmpfile.write("{} ; {}\n".format(package, marker))
+                if dist.extras_require:
+                    for extra_and_marker, packages in dist.extras_require.items():
+                        extra, marker = extra_and_marker.split(":")
+                        # TODO: non-empty extras will be ignored, perhaps we need
+                        #  an option like --with-extra to specify one. See GH-625.
+                        if extra:
+                            continue
+                        for package in packages:
+                            tmpfile.write("{} ; {}\n".format(package, marker))
             else:
                 tmpfile.write(sys.stdin.read())
             tmpfile.flush()