Fix bug in trimming USX text with partial ranges

If one part of the range to trim (say, the start chapter and verse) was
set but not another part (the end chapter and verse), the updateText
function in TextCommands would fail with an "Undefined index: endVs"
(or endCh, or whatever) error. It turns out that PHP doesn't let you use
constructions like "$array[key] || default_value" -- this does not work
like Python's "array.get(key, default_value)" but instead throws an
error. So before trying to retrieve the range values, we'll first make
sure that they're all set to a default value of 0 if they don't exist.