Improve worksheet handling in ExcelService

Updated the file header in `ExcelService.cs` to include an MIT license statement. Enhanced the logic for handling missing worksheets by selecting the first worksheet as a fallback if the specified worksheet is not found. Added error handling for cases where the workbook contains no worksheets, improving code robustness.

Author: neozhu

src/Infrastructure/Services/ExcelService.cs

@@ -1,4 +1,4 @@
-// Licensed to the .NET Foundation under one or more agreements.
+// Licensed to the .NET Foundation under one or more agreements.
 // The .NET Foundation licenses this file to you under the MIT license.
 
 using System.Data;
@@ -97,8 +97,11 @@ public async Task<IResult<IEnumerable<TEntity>>> ImportAsync<TEntity>(
         using var workbook = new XLWorkbook(new MemoryStream(data));
         if (!workbook.Worksheets.TryGetWorksheet(sheetName, out var ws))
         {
-            var msg = $"Sheet with name {sheetName} does not exist!";
-            return await Result<IEnumerable<TEntity>>.FailureAsync(msg);
+            ws = workbook.Worksheets.Count > 0 ? workbook.Worksheets.First() : null;
+            if (ws is null)
+            {
+                return await Result<IEnumerable<TEntity>>.FailureAsync("Workbook contains no worksheets.");
+            }
         }
 
         // Check if the worksheet contains any cells.