fix: initialize edge_property_type value in adjacency_list.hpp

[Becheler]

Refs #496

Before submitting

• This PR targets the develop branch.
• I searched for an existing PR or issue covering the same change.
• My contribution is licensed under the Boost Software License 1.0.

Type of change

• Bug fix
• New feature or API addition
• Refactor (no behavior change)
• Documentation
• Build, CI, or tooling
• Other (specify below)

Does this PR introduce a breaking change?

• Yes (describe migration impact below)
• No

What this PR does

Motivation

Fix #496

Testing

Same CI, less warnings

Checklist

• Existing tests pass (b2 in the test/ directory).
• New behavior is covered by a test, or this is a docs / build / refactor change.
• Documentation was updated if user-facing behavior changed.
• No new compiler warnings on the platforms I built against.

[github-actions]

Compiler-warning counts vs develop (auto-generated).
PR run 27123410669 vs develop run 27120109824 (d86a207a6b).

Job	Baseline	After	Delta
macos (clang, 14)	75	73	-2
macos (clang, 17)	70	68	-2
macos (clang, 20)	70	68	-2
ubuntu (clang-19, 14)	75	73	-2
ubuntu (clang-19, 17)	75	73	-2
ubuntu (clang-19, 20)	75	73	-2
ubuntu (clang-19, 23)	75	73	-2
ubuntu (gcc-14, 14)	51	46	-5
ubuntu (gcc-14, 17)	51	46	-5
ubuntu (gcc-14, 20)	51	46	-5
ubuntu (gcc-14, 23)	51	46	-5
windows_msvc_14_3 (msvc-14.3)	1226	1226	0