Filter empty string in PingTestGatewayIPsMap yagl

When using `ManageNetworks: false` the gateway IPs are
empty string instead of `null` when not set. The YAQL
expression filters `null` values, but the empty string
value is included in the list. The ping gateway test end
up trying to run "ping $args $empty_string" which fails.

This change improves the yaql expression to also filter
strings with 0 lenght.

Closes-Bug: #1973866
Change-Id: I7d8712223d077ab8e25239b891bd03a1324e01a8
(cherry picked from commit 7f6a4b4)

overcloud.j2.yaml

@@ -1201,7 +1201,7 @@ resources:
 {%- for role in roles %}
         {{role.name}}:
           yaql:
-            expression: list($.data.where($ != null)).flatten()
+            expression: list($.data.where($ != null and len($) > 0)).flatten()
             data:
               - {get_attr: [Networks, net_attributes_map, ctlplane, subnets, {get_param: {{role.name}}ControlPlaneSubnet}, gateway_ip]}
     {%- for network in networks %}

releasenotes/notes/fix-PingTestGatewayIPsMap-yaql-filter-empty-string-b024ce1bce9754ef.yaml

@@ -0,0 +1,11 @@
+---
+fixes:
+  - |
+    Fixes an issue where gateway ping validations performed during deployment
+    would fail. When setting the `ManageNetworks` parameter to `false` and no
+    gateway was configured, the list of gateway IP addresses to ping would
+    include empty strings for networks with no gateway. The validation would
+    attempt to run a ping command without the address to ping, which caused the
+    deployment to fail. See bug:
+    `1973866 <https://bugs.launchpad.net/tripleo/+bug/1973866>`_.
+