Fix segmentation fault with empty array

[tautschnig]

We must not attempt to access the first operand of an array expression
when the array is empty. This case is explicitly considered earlier in
this method (an empty array is considered uniform), but value equality
constraints were attempted nevertheless.

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[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 282f5d5).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/113346499