When there's a trailing space in the cmdline from the boot loader, this
results in an empty string being emitted from `String::split` after
splitting apart the argument list. This empty string resulted in a
zero-length Vector from the subsequent call to split the key=value pairs,
which was unexpected. This ultimately caused a crash when we tried to
access `[0]` of that zero-length vector.
We now detect and handle an empty string coming from `String::split`
correctly.
