Just like assigning a nonempty value, assigning an empty value to a
shell variable when calling a function produces non-portable behavior:
in some shells, the assignment lasts for the duration of the function
invocation, and in others, it persists after the function returns.

Signed-off-by: Jonathan Nieder <redacted>
---

diff --git a/t/check-non-portable-shell.pl b/t/check-non-portable-shell.pl

@@ -46,7 +46,7 @@ sub err {
-       /^\s*([A-Z0-9_]+=(\w+|(["']).*?\3)\s+)+(\w+)/ and exists($func{$4}) and
+       /^\s*([A-Z0-9_]+=(\w*|(["']).*?\3)\s+)+(\w+)/ and exists($func{$4}) and
                err '"FOO=bar shell_func" assignment extends beyond "shell_func"';