Fix `collectEmptyEqs()` in string utilsHEADmaster

Fixes #6483. The benchmark in the issue was performing the following
incorrect rewrite:

```
 Rewrite (str.replace "B" (str.replace (str.++ (str.replace "B" a "B") a) "B" "") "B") to (str.replace "B" a "B") by RPL_X_Y_X_SIMP.
```

The rewrite `RPL_X_Y_X_SIMP` rewrites terms of the form `(str.replace x
y x)`, where `x` is of length one and `(= y "")` rewrites to a
conjunction of equalities of the form `(= y_i "")` where `y_i` is some
term. The function responsible for collecting the terms `y_i` from this
conjunction, `collectEmptyEqs()`, returns a `bool` and a vector of
`Node`s. The `bool` indicates whether all equalities in the conjunction
were of the form `(= y_i "")`. The rewrite `RPL_X_Y_X_SIMP` only applies
if this is true. However, `collectEmptyEqs()` had a bug where it would
not return false when all of the conjuncts were equalities but not all
of them were equalities with the empty string. This commit fixes
`collectEmptyEqs()` and adds tests.

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