script/expect-post-runner.sh: Don't error out if UART plans don't exist

Handle the case when `$WORKSPACE/artefacts/debug/run/uart*` doesn't expand
to any file(s).

Signed-off-by: Paul Sokolovsky <paul.sokolovsky@linaro.org>
Change-Id: I44001f90b45cbbd0572fb073a1db344dc35a50ce
diff --git a/script/expect-post-runner.sh b/script/expect-post-runner.sh
index c41851e..ab22562 100755
--- a/script/expect-post-runner.sh
+++ b/script/expect-post-runner.sh
@@ -20,6 +20,11 @@
 failed=0
 
 for uartdir in $WORKSPACE/artefacts/debug/run/uart*; do
+    # In case no dirs exist and the glob above isn't expanded at all.
+    if [ ! -d "$uartdir" ]; then
+        break
+    fi
+
     uart=$(basename $uartdir)
     if [ $uart == "uart0" ]; then
         continue