LinkName setter null check.

src/libraries/System.Formats.Tar/src/System/Formats/Tar/TarEntry.cs

@@ -124,6 +124,7 @@ public string LinkName
                 {
                     throw new InvalidOperationException(SR.TarEntryHardLinkOrSymLinkExpected);
                 }
+                ArgumentNullException.ThrowIfNull(value);
                 _header._linkName = value;
             }
         }

src/libraries/System.Formats.Tar/tests/TarTestsBase.cs

@@ -299,6 +299,7 @@ protected void SetCommonSymbolicLink(TarEntry symbolicLink)
 
             // LinkName
             Assert.Equal(DefaultLinkName, symbolicLink.LinkName);
+            Assert.Throws<ArgumentNullException>(() => symbolicLink.LinkName = null);
             symbolicLink.LinkName = TestLinkName;
         }