When querying all_tab_columns in Oracle, the values NULL and 'NULL' (as well as all variants with different case) signify a default value of NULL. Only a data_default of '''NULL''' (as well as all variants with different case) represents a string default value, and thus an anomaly. See attached null_default_example.sql for examples.
A special case for Oracle in DbAnalyzer.getDefaultNullStringColumns could solve this.
Tested on Oracle 11g