fastboot: Don't give parsing error for unsupported 'getvar erase-block-size'.

Couldn't parse erase-block-size '0x'.
Couldn't parse logical-block-size '0x'.
mke2fs 1.43.3 (04-Sep-2016)
...

Some bootloaders explicitly return error on failed query (e.g. walleye),
while others return empty message (e.g. bullhead). We should just return
zero for empty message, instead of giving a misleading parsing error.

Test: `fastboot format userdata` on bullhead, with no parsing error.
Test: `fastboot format userdata` on walleye still works.
Change-Id: I194d41d24679929eccc65bfe895d6ab2a26315b7