Don't include empty-string arguments in compile_commands.json

compdb.go was incorrectly splitting compiler arguments. This could
cause empty strings to be included in compile_commands.json,
potentially confusing tools.

Bug: 117124308
Test: m checkbuild
Test: m SOONG_GEN_COMPDB=1 \
        SOONG_GEN_COMPDB_DEBUG=1 \
        SOONG_LINK_COMPDB_TO=$ANDROID_BUILD_TOP nothing
      Examine $ANDROID_BUILD_TOP/compile_commands.json
Change-Id: I375baf255c50a1329cd644ac584d200aba9daa69