information_schema is full of NULLs after database import
mysql doesn't find a LEFT JOIN with both tables.
You can use:
SELECT kcu.*,tc.* FROM information_schema.KEY_COLUMN_USAGE kcu
LEFT JOIN  information_schema.TABLE_CONSTRAINTS tc ON tc.CONSTRAINT_NAME = kcu.CONSTRAINT_NAME
WHERE tc.CONSTRAINT_TYPE = 'FOREIGN KEY' 
AND tc.TABLE_SCHEMA = 'testdb';

SELECT kcu.*,tc.* FROM information_schema.TABLE_CONSTRAINTS tc 
RIGHT JOIN information_schema.KEY_COLUMN_USAGE kcu ON tc.CONSTRAINT_NAME = kcu.CONSTRAINT_NAME
WHERE tc.CONSTRAINT_TYPE = 'FOREIGN KEY' 
AND tc.TABLE_SCHEMA = 'testdb'

I think it is a bug, which you can report, or else tell me and i report it