protected void merge(SchemaAndValue latest) {
            if (latest != null && latest.schema() != null && compatible) {
                if (common == null) {
                    // This null means any type is valid, so choose the new schema.
                    common = latest.schema();
                } else {
                    // There is a previous type restriction, so merge the new schema into the old one.
                    common = mergeSchemas(common, latest.schema());
                    // If there isn't a common schema any longer, then give up on finding further compatible schemas.
                    compatible = common != null;
                }
            }
        }
Flag to determine the meaning of the null sentinel in {@link #common}
If true, null means "any optional type", as no non-null values have appeared.
If false, null means "no common type", as one or more non-null values had mutually exclusive schemas.
connect/api/src/main/java/org/apache/kafka/connect/data/Values.java