fix: validate non-empty sequence IDs in FASTA and FASTQ writers

Adds a pre-processing guard that checks for empty sequence identifiers before formatting. This prevents malformed FASTA output and stops downstream processing of invalid FASTQ data by terminating early. The check is placed before existing sequence-length validations to enforce non-empty IDs during batch processing.

pkg/obiformats/fastseq_write_fasta.go

@@ -90,6 +90,9 @@ func FormatFastaBatch(batch obiiter.BioSequenceBatch, formater FormatHeader, ski
 	log.Debugf("FormatFastaBatch: #%d : %d seqs", batch.Order(), batch.Len())
 
 	for _, seq := range batch.Slice() {
+		if len(seq.Id()) == 0 {
+			log.Fatalf("Sequence identifier is empty")
+		}
 		if seq.Len() > 0 {
 			// Write header directly into bs — no intermediate string
 			bs.WriteByte('>')

pkg/obiformats/fastseq_write_fastq.go

@@ -64,6 +64,9 @@ func FormatFastqBatch(batch obiiter.BioSequenceBatch,
 	first := true
 
 	for _, seq := range batch.Slice() {
+		if len(seq.Id()) == 0 {
+			log.Fatalf("Sequence identifier is empty")
+		}
 		if seq.Len() > 0 {
 			_formatFastq(&bs, seq, formater)