As of 1.9, you are able to fill cauldrons with lava. You can do /setblock ~ ~ ~ cauldron 9 - 15 to place cauldrons filled with lava via commands.

However, there is a duplicate block called lava_cauldron.

/setblock ~ ~ ~ lava_cauldron 1 - 7 places a cauldron filled with a water that uses the lava texture. Using that weird water (filling bottles, stopping fire) causes it to turn into normal water.

lava_cauldron 9 - 15 are just duplicates of cauldron 9 - 15

lava_cauldron 0 and 8 are empty cauldrons, but they cannot be filled by the rain.

The players should not be given access to this block, since the lava cauldron can already be achieved by using the default cauldron.