Allow `parseString()` to return an empty `StringRef` when the delimiter
appears at position 0. This enables parsing pre-aggregated profile
addresses with an omitted buildid but preserved colon (`:addr` format),
where the empty buildid corresponds to the main binary.
Previously, `parseString()` rejected zero-length fields by treating
`StringEnd == 0` the same as `StringRef::npos` (delimiter not found).
These are distinct situations: `npos` means no delimiter exists, while
`0` means the field before the delimiter is empty. The fix removes the
`StringEnd == 0` sub-condition so only the missing-delimiter case
errors.
The existing test for buildid-prefixed addresses is extended to also
verify that `:addr` input produces identical output to the plain-address
and non-empty-buildid variants.
Test Plan:
Added empty-buildid input file and extended
`pre-aggregated-perf-buildid.test` to run perf2bolt with `:addr` format
and diff the fdata output against the existing buildid-prefixed result.
a8cf1a0352