Expression initialiser: do not generate nil for empty union

[tautschnig]

Empty unions do not have a meaningful union_exprt as an initialiser, and
thus the expression initialiser should make use of returning an
optionalt{} instead.

This test was generated using C-Reduce based on an example initially
generated by CSmith.

• Each commit message has a non-empty body, explaining why the change was made.
• n/a Methods or procedures I have added are documented, following the guidelines provided in CODING_STANDARD.md.
• n/a The feature or user visible behaviour I have added or modified has been documented in the User Guide in doc/cprover-manual/
• Regression or unit tests are included, or existing tests cover the modified code (in this case I have detailed which ones those are in the commit message).
• n/a My commit message includes data points confirming performance improvements (if claimed).
• My PR is restricted to a single feature or bugfix.
• n/a White-space or formatting changes outside the feature-related changed lines are in commits of their own.

[codecov]

Codecov Report

Merging #5667 (8cf524f) into develop (c29b69a) will decrease coverage by 0.02%.
The diff coverage is 82.75%.

@@             Coverage Diff             @@
##           develop    #5667      +/-   ##
===========================================
- Coverage    74.44%   74.41%   -0.03%     
===========================================
  Files         1446     1447       +1     
  Lines       157778   157786       +8     
===========================================
- Hits        117456   117420      -36     
- Misses       40322    40366      +44     

Impacted Files	Coverage Δ
jbmc/src/janalyzer/janalyzer_main.cpp	100.00% <ø> (ø)
jbmc/src/java_bytecode/assignments_from_json.cpp	97.26% <ø> (ø)
jbmc/src/java_bytecode/ci_lazy_methods.cpp	99.47% <ø> (ø)
jbmc/src/java_bytecode/ci_lazy_methods.h	100.00% <ø> (ø)
jbmc/src/java_bytecode/ci_lazy_methods_needed.cpp	98.24% <ø> (ø)
jbmc/src/java_bytecode/ci_lazy_methods_needed.h	100.00% <ø> (ø)
jbmc/src/java_bytecode/convert_java_nondet.cpp	81.15% <ø> (ø)
...java_bytecode/create_array_with_type_intrinsic.cpp	97.67% <ø> (ø)
jbmc/src/java_bytecode/expr2java.cpp	86.61% <ø> (ø)
jbmc/src/java_bytecode/expr2java.h	100.00% <ø> (ø)
... and 705 more

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[tautschnig]

Closing as #6408 addresses the issue more properly, and all tests from this PR pass as well with those changes made.