Work on code review comments

- remove `diag_info_ptr` from `IsValidStringFieldArgs` checks

Author: alinaliBQ

cpp/src/arrow/flight/sql/odbc/odbc_api.cc

@@ -69,7 +69,7 @@ inline bool IsValidStringFieldArgs(SQLPOINTER diag_info_ptr, SQLSMALLINT buffer_
                                    SQLSMALLINT* string_length_ptr, bool is_unicode) {
   const SQLSMALLINT char_size = is_unicode ? GetSqlWCharSize() : sizeof(char);
   const bool has_valid_buffer =
-      diag_info_ptr && buffer_length >= 0 && buffer_length % char_size == 0;
+      buffer_length == SQL_NTS || (buffer_length >= 0 && buffer_length % char_size == 0);
 
   // regardless of capacity return false if invalid
   if (diag_info_ptr && !has_valid_buffer) {
@@ -91,7 +91,6 @@ SQLRETURN SQLGetDiagField(SQLSMALLINT handle_type, SQLHANDLE handle,
                    << ", buffer_length: " << buffer_length << ", string_length_ptr: "
                    << static_cast<const void*>(string_length_ptr);
   // GH-46575 TODO: Add tests for SQLGetDiagField
-  using arrow::flight::sql::odbc::Diagnostics;
   using ODBC::GetStringAttribute;
   using ODBC::ODBCConnection;
   using ODBC::ODBCDescriptor;
@@ -109,7 +108,7 @@ SQLRETURN SQLGetDiagField(SQLSMALLINT handle_type, SQLHANDLE handle,
   // If buffer length derived from null terminated string
   if (diag_info_ptr && buffer_length == SQL_NTS) {
     const wchar_t* str = reinterpret_cast<wchar_t*>(diag_info_ptr);
-    buffer_length = wcslen(str) * arrow::flight::sql::odbc::GetSqlWCharSize();
+    buffer_length = wcslen(str) * GetSqlWCharSize();
   }
 
   // Set character type to be Unicode by default
@@ -164,7 +163,7 @@ SQLRETURN SQLGetDiagField(SQLSMALLINT handle_type, SQLHANDLE handle,
       return SQL_SUCCESS;
     }
 
-    // TODO implement return code function
+    // Driver manager implements SQL_DIAG_RETURNCODE
     case SQL_DIAG_RETURNCODE: {
       return SQL_SUCCESS;
     }
@@ -420,6 +419,10 @@ SQLRETURN SQLGetDiagRec(SQLSMALLINT handle_type, SQLHANDLE handle, SQLSMALLINT r
     // The length of the sql state is always 5 characters plus null
     SQLSMALLINT size = 6;
     const std::string& state = diagnostics->GetSQLState(record_index);
+
+    // Microsoft documentation does not mention
+    // any SQLGetDiagRec return value that is associated with `sql_state` buffer, so
+    // the return value for writing `sql_state` buffer is ignored by the driver.
     GetStringAttribute(is_unicode, state, false, sql_state, size, &size, *diagnostics);
   }
 
@@ -429,6 +432,10 @@ SQLRETURN SQLGetDiagRec(SQLSMALLINT handle_type, SQLHANDLE handle, SQLSMALLINT r
 
   if (message_text || text_length_ptr) {
     const std::string& message = diagnostics->GetMessageText(record_index);
+
+    // According to Microsoft documentation,
+    // SQL_SUCCESS_WITH_INFO should be returned if `*message_text` buffer was too
+    // small to hold the requested diagnostic message.
     return GetStringAttribute(is_unicode, message, false, message_text, buffer_length,
                               text_length_ptr, *diagnostics);
   }