Fixes panic on Go instrumentation

On ELF parse error, we return the list of missing fields, assuming that
we would stop the instrumentation. There is an edge case error on which
the missing fields set is empty so Beyla keeps with the instrumentation,
but the returned list of fields is nil.

pkg/internal/goexec/structmembers.go

@@ -391,7 +391,7 @@ func structMemberOffsetsFromDwarf(data *dwarf.Data) (FieldOffsets, map[GoOffset]
 		log.Debug("inspecting fields for struct type", "type", typeName)
 		if err := readMembers(reader, structMember.fields, expectedReturns, fieldOffsets); err != nil {
 			log.Debug("error reading DWARF info", "type", typeName, "error", err)
-			return nil, expectedReturns
+			return fieldOffsets, expectedReturns
 		}
 	}
 }