Return an empty collection instead of null (#2530)

* Return an empty collection instead of null

The method `get<MemberName>OrEmpty()` for collections, either lists or
maps, was supposed to return an empty collection when the collection
was not set, instead of null, but somehow we missed to add this
expected behavior. This change fixes this, if the collection was not
set we return either `Collections.emptyList()` or
`Collections.emptyMap()` depending on the collection type instead of
`null`. The user can still find whether the value was set by using the
`Optional<...> get<MemberName>()` variant that returns en empty
optional when the collection was not set.

Author: sugmanue

smithy-trait-codegen/src/main/java/software/amazon/smithy/traitcodegen/generators/GetterGenerator.java

@@ -4,6 +4,7 @@
  */
 package software.amazon.smithy.traitcodegen.generators;
 
+import java.util.Collections;
 import java.util.Optional;
 import software.amazon.smithy.codegen.core.Symbol;
 import software.amazon.smithy.codegen.core.SymbolProvider;
@@ -136,12 +137,16 @@ public Void structureShape(StructureShape shape) {
                     // If the member targets a collection shape and is optional then generate an unwrapped
                     // getter as a convenience method as well.
                     Shape target = model.expectShape(member.getTarget());
-                    if (target.isListShape() || target.isMapShape()) {
+                    boolean isListShape = target.isListShape();
+                    if (isListShape || target.isMapShape()) {
                         writer.openBlock("public $T get$UOrEmpty() {",
                                 "}",
                                 symbolProvider.toSymbol(member),
                                 symbolProvider.toMemberName(member),
-                                () -> writer.write("return $L;", symbolProvider.toMemberName(member)));
+                                () -> writer.write("return $1L == null ? $2T.$3L() : $1L;",
+                                        symbolProvider.toMemberName(member),
+                                        Collections.class,
+                                        isListShape ? "emptyList" : "emptyMap"));
                     }
                 } else {
                     writer.openBlock("public $T get$U() {",