pkg/query: handle null pprof metadata

[tsenart]

Problem

The pprof writer assumes that mapping and function dictionary fields are always populated.

• A null mapping filename or build ID indexes an empty Arrow dictionary and can panic with index out of range [0] with length 0.
• A null function name returns function ID 0, but the writer still emits a line that refers to that ID. The resulting pprof is invalid.
• A function with one missing field loses other available function metadata.

We found these cases through Polar Signals Cloud's merged pprof export. Native gRPC and grpc-web showed the same results. The input profiles are valid before the Arrow round trip.

Change

Read each optional mapping and function field only when it is present. Create a function from any available function metadata. When no function metadata exists, keep the location and address but omit the line, because pprof cannot represent a line without a function.

The table test covers each nullable mapping and function field with both an empty dictionary and a populated peer row. It checks sample values, labels, addresses, mappings, lines, and available symbols after serialization and parsing.

Validation

• go test ./pkg/profile ./pkg/query -count=1
• go vet ./pkg/query
• make go/lint (0 issues)
• git diff --check

All pass.

[tsenart]

@parca-dev/backend-maintainers Could a maintainer approve the fork workflows and review this fix? The local package tests, go vet, and make go/lint pass. The change comes from two reproducible failures in Polar Signals Cloud's merged pprof export.