\ifempty\ifempty{arg} t \else f \fi expands the
t text if arg is the empty string, and the f text
otherwise. This macro is useful when you need to test for empty
arguments to your macros, for example:
\def\foo#1{\ifempty{#1} t \else f \fi}
Since \ifempty is not a primitive conditional, it cannot be
used in places where TeX might skip tokens “at high speed”, e.g.,
within another conditional—TeX can't match up the \if's and
\fi's.
Note that the following code
\def\empty{}
\ifempty\empty\message{empty}\else\message{not empty}\fi
will produce the message ‘not empty’.