Resolve nested placeholders with a fallback having one

This commit fixes a regression in PlaceHolderParser where it would no
longer resolve nested placeholders for a case where the fallback has a
placeholder itself.

This is due to the Part implementations and how they are structure, and
this commit makes sure that nested resolution happens consistently.

Closes gh-34020

spring-core/src/test/java/org/springframework/util/PlaceholderParserTests.java

@@ -210,6 +210,8 @@ class PlaceholderParserTests {
 		static Stream<Arguments> nestedPlaceholders() {
 			return Stream.of(
 					Arguments.of("${p6}", "v1:v2:def"),
+					Arguments.of("${p6:not-used}", "v1:v2:def"),
+					Arguments.of("${p6:${invalid}}", "v1:v2:def"),
 					Arguments.of("${invalid:${p1}:${p2}}", "v1:v2"),
 					Arguments.of("${invalid:${p3}}", "v1:v2"),
 					Arguments.of("${invalid:${p4}}", "v1:v2"),