NPE When configuring AvroSerializer

[tmgstevens]

envelope/external/kafka/src/main/java/com/cloudera/labs/envelope/kafka/serde/AvroSerializer.java

Line 58 in 6709db6

String schemaLocation = configs.get(SCHEMA_PATH_CONFIG_NAME).toString();

Code should check for nulls. Also, I guess docs might need to be clearer...