Do not precede indexed access with a dot in SpEL AST representation

Prior to this commit, if a Spring Expression Language (SpEL) expression
contained indexed access to an object, the generated AST String
representation incorrectly included a dot ('.') before the index access.

For example, 'property[0]' had a generated AST string representation of
'property.[0]'.

This commit addresses this by reworking the logic in
CompoundExpression.toStringAST().

Closes gh-30610

spring-expression/src/test/java/org/springframework/expression/spel/ParsingTests.java

@@ -43,6 +43,12 @@ class ParsingTests {
 	@Nested
 	class Miscellaneous {
 
+		@Test
+		void compoundExpressions() {
+			parseCheck("property1.property2.methodOne()");
+			parseCheck("property1[0].property2['key'].methodOne()");
+		}
+
 		@Test
 		void supportedCharactersInIdentifiers() {
 			parseCheck("#var='value'");