[dns] allow Data Length = 0 while reading a TXT record (#6961)

RFC 6763 Section 6.1 states the following:

   An empty TXT record containing zero strings is not allowed [RFC1035].
   DNS-SD implementations MUST NOT emit empty TXT records.  DNS-SD
   clients MUST treat the following as equivalent:

   o  A TXT record containing a single zero byte.
      (i.e., a single empty string.)

   o  An empty (zero-length) TXT record.
      (This is not strictly legal, but should one be received, it should
      be interpreted as the same as a single empty string.)

   o  No TXT record.
      (i.e., an NXDOMAIN or no-error-no-answer response.)

This commit reflects the second requirement.

src/core/net/dns_types.cpp

@@ -1084,7 +1084,7 @@ Error TxtRecord::ReadTxtData(const Message &aMessage,
 
     VerifyOrExit(GetLength() <= aTxtBufferSize, error = kErrorNoBufs);
     SuccessOrExit(error = aMessage.Read(aOffset, aTxtBuffer, GetLength()));
-    VerifyOrExit(VerifyTxtData(aTxtBuffer, GetLength()), error = kErrorParse);
+    VerifyOrExit(VerifyTxtData(aTxtBuffer, GetLength(), /* aAllowEmpty */ true), error = kErrorParse);
     aTxtBufferSize = GetLength();
     aOffset += GetLength();
 
@@ -1092,13 +1092,13 @@ exit:
     return error;
 }
 
-bool TxtRecord::VerifyTxtData(const uint8_t *aTxtData, uint16_t aTxtLength)
+bool TxtRecord::VerifyTxtData(const uint8_t *aTxtData, uint16_t aTxtLength, bool aAllowEmpty)
 {
     bool    valid          = false;
     uint8_t curEntryLength = 0;
 
     // Per RFC 1035, TXT-DATA MUST have one or more <character-string>s.
-    VerifyOrExit(aTxtLength > 0);
+    VerifyOrExit(aAllowEmpty || aTxtLength > 0);
 
     for (uint16_t i = 0; i < aTxtLength; ++i)
     {

src/core/net/dns_types.hpp

@@ -1677,13 +1677,14 @@ public:
     /**
      * This static method tests if a buffer contains valid encoded TXT data.
      *
-     * @param[in]  aTxtData    The TXT data buffer.
-     * @param[in]  aTxtLength  The length of the TXT data buffer.
+     * @param[in]  aTxtData     The TXT data buffer.
+     * @param[in]  aTxtLength   The length of the TXT data buffer.
+     * @param[in]  aAllowEmpty  True if zero-length TXT data is allowed.
      *
      * @returns  TRUE if @p aTxtData contains valid encoded TXT data, FALSE if not.
      *
      */
-    static bool VerifyTxtData(const uint8_t *aTxtData, uint16_t aTxtLength);
+    static bool VerifyTxtData(const uint8_t *aTxtData, uint16_t aTxtLength, bool aAllowEmpty);
 
 } OT_TOOL_PACKED_END;
 

src/core/net/srp_server.cpp

@@ -1526,7 +1526,7 @@ Error Server::Service::Description::SetTxtDataFromMessage(const Message &aMessag
     VerifyOrExit(txtData != nullptr, error = kErrorNoBufs);
 
     VerifyOrExit(aMessage.ReadBytes(aOffset, txtData, aLength) == aLength, error = kErrorParse);
-    VerifyOrExit(Dns::TxtRecord::VerifyTxtData(txtData, aLength), error = kErrorParse);
+    VerifyOrExit(Dns::TxtRecord::VerifyTxtData(txtData, aLength, /* aAllowEmpty */ false), error = kErrorParse);
 
     Instance::HeapFree(mTxtData);
     mTxtData   = txtData;