Fix AbpStringExtensions

For now, `"".RemovePreFix("x")` will return null.
It's not a expected result. I think we shouldn't make empty string be null when trim prefix or postfix.

framework/src/Volo.Abp.Core/System/AbpStringExtensions.cs

@@ -133,7 +133,7 @@ namespace System
         {
             if (str.IsNullOrEmpty())
             {
-                return null;
+                return str;
             }
 
             if (postFixes.IsNullOrEmpty())
@@ -174,7 +174,7 @@ namespace System
         {
             if (str.IsNullOrEmpty())
             {
-                return null;
+                return str;
             }
 
             if (preFixes.IsNullOrEmpty())