Add a changelog entry.

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+Changed: IndexSet objects could only be compared for equality or
+inequality against other IndexSet objects that had the same size. This
+did not allow for comparison against default-constructed objects, for
+example to test whether an object had been initialized. The
+restriction is therefore relaxed: IndexSet objects can be compared for
+equality and inequality against objects of the same size, or or size
+zero.
+<br>
+(Wolfgang Bangerth, 2023/09/07)