PR feedback: use ReadOnlySpan<char>.Empty

Author: paulirwin

src/Lucene.Net.Analysis.Common/Analysis/Util/CharArrayMap.cs

@@ -3827,7 +3827,7 @@ internal static CharReturnType ConvertObjectToChars<T>(T key, out char[] chars,
 
 #if FEATURE_SPANFORMATTABLE
             else if (key is ISpanFormattable spanFormattable &&
-                spanFormattable.TryFormat(reuse, out int charsWritten, string.Empty, invariant))
+                spanFormattable.TryFormat(reuse, out int charsWritten, ReadOnlySpan<char>.Empty, invariant))
             {
                 chars = reuse.Slice(0, charsWritten).ToArray();
                 return CharReturnType.CharArray;