Fixing possible NPE while reading hub config file. Fixes #5575

Author: barancev

java/server/src/org/openqa/grid/internal/cli/GridHubCliOptions.java

@@ -49,7 +49,7 @@ private IDefaultProvider defaults(String json) {
     map.remove("custom");
     return optionName -> {
       String option = optionName.replaceAll("-", "");
-      return map.containsKey(option) ? map.get(option).toString() : null;
+      return map.containsKey(option) && map.get(option) != null ? map.get(option).toString() : null;
     };
   }
 

java/server/test/org/openqa/grid/e2e/misc/GridViaCommandLineTest.java

@@ -250,6 +250,7 @@ public void canStartHubUsingConfigFile() throws Exception {
         + " \"servlets\" : [],\n"
         + " \"withoutServlets\": [],\n"
         + " \"custom\": {},\n"
+        + " \"prioritizer\": null,\n"
         + " \"capabilityMatcher\": \"org.openqa.grid.internal.utils.DefaultCapabilityMatcher\",\n"
         + " \"registry\": \"org.openqa.grid.internal.DefaultGridRegistry\",\n"
         + " \"throwOnCapabilityNotPresent\": true,\n"