Do not error when LINKERD2_PROXY_INBOUND_PORTS is empty (#1478)

When the `LINKERD2_PROXY_INBOUND_PORTS` environment is set
and empty, we should not log an error message.

Signed-off-by: Kevin Leimkuhler <kleimkuhler@icloud.com>
Co-authored-by: Oliver Gould <ver@buoyant.io>
(cherry picked from commit b80af37)
Signed-off-by: Oliver Gould <ver@buoyant.io>

Author: kleimkuhler

linkerd/app/src/env.rs

@@ -969,8 +969,10 @@ fn parse_addr(s: &str) -> Result<Addr, ParseError> {
 
 fn parse_port_set(s: &str) -> Result<HashSet<u16>, ParseError> {
     let mut set = HashSet::new();
-    for num in s.split(',') {
-        set.insert(parse_number::<u16>(num)?);
+    if !s.is_empty() {
+        for num in s.split(',') {
+            set.insert(parse_number::<u16>(num)?);
+        }
     }
     Ok(set)
 }