fix(ast): removed lambda void return type check

The LambdaExpr class will now not fail if a lambda with void return
type is provided

Author: diegomarquezp

src/main/java/com/google/api/generator/engine/ast/LambdaExpr.java

@@ -67,9 +67,6 @@ public Builder setReturnExpr(Expr expr) {
 
     public LambdaExpr build() {
       LambdaExpr lambdaExpr = autoBuild();
-      Preconditions.checkState(
-          !lambdaExpr.returnExpr().expr().type().equals(TypeNode.VOID),
-          "Lambdas cannot return void-typed expressions.");
       // Must be a declaration.
       lambdaExpr.arguments().stream()
           .forEach(

src/test/java/com/google/api/generator/engine/ast/LambdaExprTest.java

@@ -85,17 +85,15 @@ public void validLambdaExpr_withBody() {
   }
 
   @Test
-  public void invalidLambdaExpr_returnsVoid() {
-    assertThrows(
-        IllegalStateException.class,
-        () ->
-            LambdaExpr.builder()
-                .setReturnExpr(
-                    MethodInvocationExpr.builder()
-                        .setMethodName("foo")
-                        .setReturnType(TypeNode.VOID)
-                        .build())
-                .build());
+  public void validLambdaExpr_returnsVoid() {
+    LambdaExpr voidLambda = LambdaExpr.builder()
+        .setReturnExpr(
+            MethodInvocationExpr.builder()
+                .setMethodName("foo")
+                .setReturnType(TypeNode.VOID)
+                .build())
+        .build();
+    assertEquals(TypeNode.VOID, voidLambda.returnExpr().type());
   }
 
   @Test