Add null pointer check to add/put methods (#5385) (#5386)

Commit e3d04ee01 (issue #5240) introduced unsafe add/put methods for optional
list/map parameters. This change maintains the spirit of issue #5240 (optional
containers are null by default) while still making add/put calls safe. It does
this by checking for null first and, if so, initializing it with an empty
container.

Also updated the affected samples using the various scripts in bin/.

modules/swagger-codegen/src/main/resources/JavaSpring/pojo.mustache

@@ -43,6 +43,11 @@ public class {{classname}} {{#parent}}extends {{{parent}}}{{/parent}} {{#seriali
   {{#isListContainer}}
 
   public {{classname}} add{{nameInCamelCase}}Item({{{items.datatypeWithEnum}}} {{name}}Item) {
+    {{^required}}
+    if (this.{{name}} == null) {
+      this.{{name}} = {{{defaultValue}}};
+    }
+    {{/required}}
     this.{{name}}.add({{name}}Item);
     return this;
   }
@@ -50,6 +55,11 @@ public class {{classname}} {{#parent}}extends {{{parent}}}{{/parent}} {{#seriali
   {{#isMapContainer}}
 
   public {{classname}} put{{nameInCamelCase}}Item(String key, {{{items.datatypeWithEnum}}} {{name}}Item) {
+    {{^required}}
+    if (this.{{name}} == null) {
+      this.{{name}} = {{{defaultValue}}};
+    }
+    {{/required}}
     this.{{name}}.put(key, {{name}}Item);
     return this;
   }