Don't append empty component to start_dir

In most cases `start_dir` is unset/empty.
When passing it to `os.path.join` it leads to `/topdir/` which could
case issues when using the template as `%(startdir)s/subdir`
as a double-slash might be treated specially.

Author: Flamefire

easybuild/framework/easyblock.py

@@ -2368,7 +2368,7 @@ def guess_start_dir(self):
                 # make sure start_dir subdir exists (cfr. check below)
                 mkdir(os.path.join(topdir, start_dir), parents=True)
 
-            abs_start_dir = os.path.join(topdir, start_dir)
+            abs_start_dir = os.path.join(topdir, start_dir) if start_dir else topdir
             if topdir.endswith(start_dir) and not os.path.exists(abs_start_dir):
                 self.cfg['start_dir'] = topdir
             else:

test/framework/easyblock.py

@@ -2845,6 +2845,8 @@ def check_start_dir(expected_start_dir):
             abs_expected_start_dir = os.path.join(eb.builddir, expected_start_dir)
             self.assertTrue(os.path.samefile(eb.cfg['start_dir'], abs_expected_start_dir))
             self.assertTrue(os.path.samefile(os.getcwd(), abs_expected_start_dir))
+            self.assertTrue(eb.cfg['start_dir'].endswith(abs_expected_start_dir))
+            self.assertFalse(eb.cfg['start_dir'].endswith(os.path.sep))
 
         # default (no start_dir specified): use unpacked dir as start dir
         self.assertEqual(ec['ec']['start_dir'], None)