NIFI-7297 add available()

Signed-off-by: Pierre Villard <pierre.villard.fr@gmail.com>

This closes #4172.

nifi-mock/src/main/java/org/apache/nifi/util/MockProcessSession.java

@@ -618,6 +618,11 @@ public class MockProcessSession implements ProcessSession {
                 bais.reset();
             }
 
+            @Override
+            public int available() throws IOException {
+                return bais.available();
+            }
+
             @Override
             public String toString() {
                 return "ErrorHandlingInputStream[flowFile=" + mock + "]";