Adds a name/value pair to the formatted output in name=value format. If value
is null, the string "null" is used, unless #omitNullValues() is
called, in which case this name/value pair will not be added.
Configures the ToStringHelper so #toString() will ignore properties with null
value. The order of calling this method, relative to the add()/addValue()
methods, is not significant.