SpEL's `CompoundExpression.toStringAST()` incorrectly includes `.` for indexed properties

[sbrannen]

I discovered this bug while writing tests for gh-30580.

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

Line 74 in aa2a067

parseCheck("begrüssung[1]", "begrüssung.[1]"); // extra "." is needed due to erroneous logic in CompoundExpression.toStringAST()

Essentially, an expression like property1[0].property2['key'].methodOne() should have an identical AST string representation; whereas, the current AST string representation is property1.[0].property2.['key'].methodOne() with a . preceding every indexed property access.