Collapse the two id emptiness tests into one and pin the reassembler wording

AGENTS.md

@@ -207,7 +207,7 @@ exactly 140.
   `esm_class` at 0, and reading that as the spec's "default message type" would lose every one of
   them. A non-empty `receipted_message_id` TLV marks a receipt on the same footing there, since
   nothing but a receipt carries one. What gets scraped is the decoded `short_message` with any UDH
-  stripped; receipts never reach the reassembler, so an SMSC that splits one across segments gets a
+  stripped; a receipt this library recognises never reaches the reassembler, so an SMSC that splits one across segments gets a
   `dlr` per segment rather than one merged report.
   The `message_state` TLV is authoritative only where it names a state in the table — SMPP reserves
   0x80-0xFF for MC-vendor-specific values, so an unnameable one keeps its raw `statusId` and leaves

src/dlr.ts

@@ -160,7 +160,7 @@ function receiptBody(pduObj: PduObject): string {
 }
 
 function receiptId(tlvId: ParamValue | undefined, receipt: Receipt | undefined): string | undefined {
-	return nonEmptyText(tlvId) ?? (receipt?.id === '' ? undefined : receipt?.id);
+	return nonEmptyText(tlvId) ?? nonEmptyText(receipt?.id);
 }
 
 function isMessageState(name: string | undefined): name is MessageState {

test/dlr.test.ts

@@ -117,7 +117,7 @@ describe('dlrFromPdu()', () => {
 		assert.equal(dlrFromPdu(deliverSm('id:x stat:DELIVRD done date:2501012560'))?.doneDate, undefined);
 	});
 
-	test('returns nothing when an unmarked deliver_sm identifies no message', () => {
+	test('returns nothing when an unmarked deliver_sm yields no id or no status', () => {
 		assert.equal(dlrFromPdu(deliverSm('just a normal sms', undefined, 0)), undefined);
 		assert.equal(dlrFromPdu(deliverSm('id:0195f0c7 stat:WEIRDXX', undefined, 0)), undefined);
 	});
@@ -180,8 +180,6 @@ describe('dlrFromPdu()', () => {
 		assert.equal(empty, undefined, 'an empty id marks nothing');
 	});
 
-	// The spec names six of the sixteen message types and reserves the rest, so a peer that types
-	// its receipts with one of the reserved bits keeps the body scrape rather than losing them.
 	test('keeps the body scrape for a message type the spec reserves', () => {
 		const dlr = dlrFromPdu(deliverSm(receiptText, undefined, 0x0c));