ro.boot.dynamic_partitions: fix empty values

When PRODUCT_RETROFIT_DYNAMIC_PARTITIONS is empty, collapse-pairs will
write this line to build.prop:
ro.boot.dynamic_partitions=ro.boot.dynamic_partitions_retrofit=

... which is not what we want.
Test: build without dynamic partitions

Change-Id: I04c3c75fad20604657814b7f9cce2cc0635627ee