fix edge case

Author: inemtsev

modules/openapi-generator/src/main/resources/Java/libraries/native/ApiClient.mustache

@@ -570,6 +570,9 @@ public class ApiClient {
 
     @Override
     public int read(byte[] b, int off, int len) throws IOException {
+      if (len == 0) {
+        return 0;
+      }
       if (!fillBuffer()) {
         return -1;
       }

samples/client/petstore/java/native-useGzipFeature/src/main/java/org/openapitools/client/ApiClient.java

@@ -562,6 +562,9 @@ public int read() throws IOException {
 
     @Override
     public int read(byte[] b, int off, int len) throws IOException {
+      if (len == 0) {
+        return 0;
+      }
       if (!fillBuffer()) {
         return -1;
       }