Why is the value of NUMERIC_SCALE column in INFORMATION_SCHEMA.COLUMNS table for DATA_TYPE='double' NULL?
Floating point numbers are exactly that -- there's no fixed position for the decimal separator (point), it can be anywhere, and specifying scale (i.e. the number of digits after the decimal separator) does not make sense. 

Scale is used for the DECIMAL data type, a type with the fixed decimal separator, for which scale is significant and meaningful.

MySQL also allows a non-standard syntax for floating point numbers, where you can specify precision and scale, e.g. DOUBLE(10, 5). MySQL will round floating point values to that specification when storing them, and format them appropriately when returning query results.



Documentation link.

dbfiddle example.