tracing/histogram: Fix check for missing operands in an expression

tracing/histogram: Fix check for missing operands in an expression

If a binary operation is detected while parsing an expression string,
the operand strings are deduced by splitting the experssion string at
the position of the detected binary operator. Both operand strings are
sub-strings (can be empty string) of the expression string but will
never be NULL.

Currently a NULL check is used for missing operands, fix this by
checking for empty strings instead.

Link: https://lkml.kernel.org/r/20211112191324.1302505-1-kaleshsingh@google.com
Signed-off-by: Kalesh Singh <kaleshsingh@google.com>
Fixes: dfdca7209867 ("tracing: Fix operator precedence for hist triggers expression")
Reported-by: Dan Carpenter <dan.carpenter@oracle.com>
Signed-off-by: Steven Rostedt (VMware) <rostedt@goodmis.org>