[SPARK-48584][SQL][FOLLOWUP] Improve the unescapePathName

### What changes were proposed in this pull request?
This PR follows up #46938 and improve the `unescapePathName`.

### Why are the changes needed?
Improve the `unescapePathName` by cut off slow path.

### Does this PR introduce _any_ user-facing change?
'No'.

### How was this patch tested?
GA.

### Was this patch authored or co-authored using generative AI tooling?
'No'.

Closes #46957 from beliefer/SPARK-48584_followup.

Authored-by: beliefer <beliefer@163.com>
Signed-off-by: beliefer <beliefer@163.com>

sql/catalyst/src/main/scala/org/apache/spark/sql/catalyst/catalog/ExternalCatalogUtils.scala

@@ -103,7 +103,7 @@ object ExternalCatalogUtils {
     }
     var plaintextEndIdx = path.indexOf('%')
     val length = path.length
-    if (plaintextEndIdx == -1 || plaintextEndIdx + 2 > length) {
+    if (plaintextEndIdx == -1 || plaintextEndIdx + 2 >= length) {
       // fast path, no %xx encoding found then return the string identity
       path
     } else {

sql/catalyst/src/test/scala/org/apache/spark/sql/catalyst/catalog/ExternalCatalogUtilsSuite.scala

@@ -59,6 +59,8 @@ class ExternalCatalogUtilsSuite extends SparkFunSuite {
     assert(unescapePathName("a%2Fb") === "a/b")
     assert(unescapePathName("a%2") === "a%2")
     assert(unescapePathName("a%F ") === "a%F ")
+    assert(unescapePathName("%0") === "%0")
+    assert(unescapePathName("0%") === "0%")
     // scalastyle:off nonascii
     assert(unescapePathName("a\u00FF") === "a\u00FF")
     // scalastyle:on nonascii