Fix collapse-pairs for last empty property

If the last token in the argument for collapse-pairs is
key=, then the following lines were written:
    key
    =
This patch fixes it so that key= is written instead.

Test: specify ro.boot.logical_partitions= at the end of
ADDITIONAL_PRODUCT_PROPERTIES and `cat $OUT/product/build.prop`
shows:
ro.boot.logical_partitions=

Change-Id: I33a0efb59871f0e36a93c8fae0d75f2735994301