Enforce validation for PathHierarchy tokenizer (#23510)

If delimiter or replacement parameter are an empty string, the error is not clear enough to indicate how to fix it.
With this change, the user knows these parameter must be a non empty string.

Author: ebuildy

core/src/main/java/org/elasticsearch/index/analysis/PathHierarchyTokenizerFactory.java

@@ -41,17 +41,17 @@ public PathHierarchyTokenizerFactory(IndexSettings indexSettings, Environment en
         String delimiter = settings.get("delimiter");
         if (delimiter == null) {
             this.delimiter = PathHierarchyTokenizer.DEFAULT_DELIMITER;
-        } else if (delimiter.length() > 1) {
-            throw new IllegalArgumentException("delimiter can only be a one char value");
+        } else if (delimiter.length() != 1) {
+            throw new IllegalArgumentException("delimiter must be a one char value");
         } else {
             this.delimiter = delimiter.charAt(0);
         }
 
         String replacement = settings.get("replacement");
         if (replacement == null) {
             this.replacement = this.delimiter;
-        } else if (replacement.length() > 1) {
-            throw new IllegalArgumentException("replacement can only be a one char value");
+        } else if (replacement.length() != 1) {
+            throw new IllegalArgumentException("replacement must be a one char value");
         } else {
             this.replacement = replacement.charAt(0);
         }