Remove null checks from Path.CombineInternal (dotnet#119922)

All the arguments are validated not null in the public constructors.

Author: xtqqczze

src/libraries/System.Private.CoreLib/src/System/IO/Path.cs

@@ -650,10 +650,10 @@ public static bool TryJoin(ReadOnlySpan<char> path1, ReadOnlySpan<char> path2, R
 
         private static string CombineInternal(string first, string second)
         {
-            if (string.IsNullOrEmpty(first))
+            if (first.Length == 0)
                 return second;
 
-            if (string.IsNullOrEmpty(second))
+            if (second.Length == 0)
                 return first;
 
             if (IsPathRooted(second.AsSpan()))
@@ -664,11 +664,11 @@ private static string CombineInternal(string first, string second)
 
         private static string CombineInternal(string first, string second, string third)
         {
-            if (string.IsNullOrEmpty(first))
+            if (first.Length == 0)
                 return CombineInternal(second, third);
-            if (string.IsNullOrEmpty(second))
+            if (second.Length == 0)
                 return CombineInternal(first, third);
-            if (string.IsNullOrEmpty(third))
+            if (third.Length == 0)
                 return CombineInternal(first, second);
 
             if (IsPathRooted(third.AsSpan()))
@@ -681,13 +681,13 @@ private static string CombineInternal(string first, string second, string third)
 
         private static string CombineInternal(string first, string second, string third, string fourth)
         {
-            if (string.IsNullOrEmpty(first))
+            if (first.Length == 0)
                 return CombineInternal(second, third, fourth);
-            if (string.IsNullOrEmpty(second))
+            if (second.Length == 0)
                 return CombineInternal(first, third, fourth);
-            if (string.IsNullOrEmpty(third))
+            if (third.Length == 0)
                 return CombineInternal(first, second, fourth);
-            if (string.IsNullOrEmpty(fourth))
+            if (fourth.Length == 0)
                 return CombineInternal(first, second, third);
 
             if (IsPathRooted(fourth.AsSpan()))