Added null check before reading from the request input stream (#147)

Author: sapessi

aws-serverless-java-container-core/src/main/java/com/amazonaws/serverless/proxy/internal/servlet/AwsProxyHttpServletRequest.java

@@ -892,6 +892,9 @@ public void setReadListener(ReadListener readListener) {
         @Override
         public int read()
                 throws IOException {
+            if (bodyStream == null || bodyStream instanceof NullInputStream) {
+                return -1;
+            }
             int readByte = bodyStream.read();
             if (bodyStream.available() == 0 && listener != null) {
                 listener.onAllDataRead();