[BUG] if server respond with 204(no content) client getting NPE here

Good day!

https://github.com/recurly/recurly-client-java/blob/9fba0d3393496ee6b269131a139ef92d7dd82a4a/src/main/java/com/recurly/v3/BaseClient.java#L147C11-L147C82

```
if (BINARY_TYPES.contains(contentType.type() + "/" + contentType.subtype())) {
```
please replace to 
```
if (contentType != null && BINARY_TYPES.contains(contentType.type() + "/" + contentType.subtype())) {
````