fix(soroban): skip empty CFGs in emit_functions_with_spec

[0xull]

Description

an unused modifier produces a CFG with no basic blocks. the Soroban
emit path passed it to emit_cfg which called create_block and indexed
into the empty block list, panicking. the generic emit_functions path
already guards against this with cfg.is_placeholder(). apply the same
guard in emit_functions_with_spec.

Fix

one guard added in src/emit/soroban/mod.rs to skip any CFG where cfg.is_placeholder() returns true before passing it to emit_cfg.

Tests

one test in tests/soroban_testcases/modifiers.rs that compiles the exact MRE from the issue and invokes run() to show the contract is not only panic-free but executable.

Fixes #1906