Avoid zero-length prefixes in PATH

A zero-length prefix in PATH is a legacy feature that indicates the
current working directory.

https://pubs.opengroup.org/onlinepubs/9699919799/basedefs/V1_chap08.html#tag_08_03

Found in ru.nsu.bobrofon.easysshfs, see:
https://gitlab.com/fdroid/fdroiddata/-/merge_requests/10953#note_921802636

fdroidserver/common.py

@@ -3006,7 +3006,7 @@ def set_FDroidPopen_env(build=None):
     if build is not None:
         path = build.ndk_path()
         paths = orig_path.split(os.pathsep)
-        if path not in paths:
+        if path and path not in paths:
             paths = [path] + paths
             env['PATH'] = os.pathsep.join(paths)
         for n in ['ANDROID_NDK', 'NDK', 'ANDROID_NDK_HOME']:

tests/common.TestCase

@@ -2364,6 +2364,14 @@ class CommonTest(unittest.TestCase):
             "%s_%s.exe" % (app.id, build.versionCode),
         )
 
+    def test_no_zero_length_ndk_path_prefixes(self):
+        fdroidserver.common.config = {'ndk_paths': {}}
+        build = fdroidserver.metadata.Build()
+
+        os.environ['PATH'] = '/usr/bin:/usr/sbin'
+        fdroidserver.common.set_FDroidPopen_env(build)
+        self.assertNotIn('', os.getenv('PATH').split(os.pathsep))
+
 
 if __name__ == "__main__":
     os.chdir(os.path.dirname(__file__))