Most efficient way to replace null values in an array with 0?

If you just need to remove the null values from array then I would just to use remove(a!isnull,<array>).

But if you really have to replace then I would suggest not to use apply. 

The following code snippet might be helpful:

a!localVariables(
  local!data: { 1, null, 0, 10, null },
  local!updatedData: a!update(
    local!data,
    wherecontains(tointeger(null()), local!data),
    0
  ),
  local!updatedData
)

Luckily the a!defaultValue() function does exactly what you're looking for! You just have to run it using a!forEach() to apply it to a list of values:

a!localVariables(
  local!data: {1, 2, null, 4, null, 6},
  a!forEach(
    items: local!data,
    expression: a!defaultValue(
      value: fv!item,
      default: 0
    )
  )
)

Also you asked about the apply() function - at this point, everything that the apply function does can also be done by a!forEach(), so I recommend using a!forEach() instead if you need to loop over a list of values.