luci-base: ensure that file upload values have length

Ensure that the (table) length of a file upload value has nonzero length
by initializing the first table index with the file name.

This fixes tests in the form

    x = luci.http.formvalue(...)
    if x and #x > 0 then ... end

Fixes #1763.
Signed-off-by: Jo-Philipp Wich <jo@mein.io>