Empty LONGTEXT or NULL?
A good starting point to make a decision in order to use or not this kind of value is the MySQL official manual.

In short:

Optimization 


  Declare columns to be NOT NULL if possible. It makes SQL operations
  faster, by enabling better use of indexes and eliminating overhead for
  testing whether each value is NULL. You also save some storage space,
  one bit per column. If you really need NULL values in your tables, use
  them. Just avoid the default setting that allows NULL values in every
  column.


Problems with NULL values


  In SQL, the NULL value is never true in comparison to any other value,
  even NULL. An expression that contains NULL always produces a NULL
  value unless otherwise indicated in the documentation for the
  operators and functions involved in the expression
  
  When reading data with LOAD DATA INFILE, empty or missing columns are
  updated with ''. To load a NULL value into a column, use \N in the
  data file