Add missing member initializers of string_constraintst-typed value [blocks: #2310]

[tautschnig]

add_axioms_for_char_at, add_axioms_for_is_empty, add_axioms_from_char only
initialized one of the three vectors.

@romainbrenguier I am not entirely sure the modified initialisation code actually initialises the intended members.

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[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: 04b016b).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/90982543

[romainbrenguier]

This seems ok, but maybe a clearer solution would be to declare the string_constrainst explicitely:

string_constraintst constraints;
constraints.existential = equal_exprt(char_sym, str[f.arguments()[1]]);
return {char_sym, constraints};

to replace the last two lines.

[romainbrenguier]

This is ok, I just put a suggestion to make the code clearer.

[allredj]

✔️
Passed Diffblue compatibility checks (cbmc commit: c078a69).
Build URL: https://travis-ci.com/diffblue/test-gen/builds/95249315