Allow bash_profile to run when unbound variable checking is enabled

Change-Id: I0086b4cc41c6206a47610b8c8e4062c30ac7dba3
diff --git a/tools/dev/bash_profile b/tools/dev/bash_profile
index 34c6b69..88fb517 100644
--- a/tools/dev/bash_profile
+++ b/tools/dev/bash_profile
@@ -167,7 +167,7 @@
 function setPrimaryInstance {
     export ONOS_INSTANCES=$(env | grep 'OC[0-9]*=' | sort | cut -d= -f2)
 
-    if [ -z "$OCC1" ]; then
+    if [ -z "${OCC1-}" ]; then
         aux=/tmp/cell-$$.aux
         let i=1
         for n in $ONOS_INSTANCES; do