babelfish-for-postgresql / babelfish_extensions / 8479429136

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String empty check in PLtsql_expr_query_mutator (#2432)

Add a check that string is not empty before calling `.front()` on it (that is apparently [an UB on empty string](https://en.cppreference.com/w/cpp/string/basic_string/front)).

There are many usages of `.front()` like this in `tsqlIface.cpp`, but AFAICS they always operate on known non-empty strings. Not sure if more non-empty checks before `.front()` calls are necessary there. Such crashes are deterministic with `-O0`, I've run JDBC test suite with `-O0` server and didn't get any more crashes.

### Issues Resolved

https://github.com/babelfish-for-postgresql/babelfish_extensions/issues/2431

### Test Scenarios Covered ###

Without patch the problem is reproducible with [AVG-Aggregate-common-vu-verify](https://github.com/babelfish-for-postgresql/babelfish_extensions/blob/b38e6e2c0/test/JDBC/input/AVG-Aggregate-common-vu-verify.sql#L65) existing test (and probably some others).

Signed-off-by: Alex Kasko <alex@staticlibs.net>

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