Most of the time a block of code is empty when a piece of code is really missing. So such empty block must be either filled or removed.

Noncompliant Code Example

for (int i = 0; i < 42; i++){}  // Empty on purpose or missing piece of code ?

Exceptions

When a block contains a comment, this block is not considered to be empty.

while and unless loops are also exception to the rule.

while (order.processNext());  // Compliant