Provide our container classes with empty() functions.

[bangerth]

All of the C++ container classes seem to have received empty() functions that make writing x.size() == 0 simpler. We might want to think of adding those to our classes as well. Currently, we have

> grep -r 'size() == 0' include/ source/ examples/ | wc -l
118

All of these 118 places could be simplified.

[masterleinad]

See #15731 and #15661.