armor data resetting material_thickness and environmental_protection

[Saicchi]

Describe the bug

The armor fields material_thickness and environmental_protection are not copied / reset when using copy-from. Using proportional and relative on those fields cause the error: Member material_thickness of type cata::optional<float> does not support relative

After some testing I found out that if the item that is copying from the base item defines armor field, it resets / deletes the material_thickness and environmental_protection.

Steps to reproduce

1. Load up a game with Magiclysm enabled.
2. Spawn XL black dragonscale armor
3. Spawn black dragonscale armor
4. Compare both armors.

Expected behavior

material_thickness and environmental_protection being able to use copy-from, proportional and relative when armor data is defined.

Screenshots

Versions and configuration

• OS: Linux
  • OS Version: LSB Version: 1.4; Distributor ID: Arch; Description: Arch Linux; Release: rolling; Codename: n/a;
• Game Version: 0.F-2728-g98649ca0c9-dirty [64-bit]
• Graphics Version: Tiles
• Game Language: English [en]
• Mods loaded: [
  Dark Days Ahead [dda],
  Disable NPC Needs [no_npc_food],
  No Fungal Growth [no_fungal_growth],
  Bionic Professions [package_bionic_professions],
  Magiclysm [magiclysm],
  Sacchi's Debug Mod [sacchitesting]
  ]

Additional context

Though I show items from Magiclysm, this is not a Magiclysm specific bug, it occurs with other items in vanilla too.
For example, add "armor": [ { "encumbrance": 10, "coverage": 90, "covers": [ "torso", "leg_l", "leg_r" ] } ] to xl_armor_chitin which uses the same value as the item it's copying from armor_chitin, its values will be reset.

[dseguin]

Possible fix (untested, but I have a hunch): Edit: Nope, I'm a dummy. Didn't work.

diff --git a/src/item_factory.cpp b/src/item_factory.cpp
index 7c39bb2db7..cd1151b6d7 100644
--- a/src/item_factory.cpp
+++ b/src/item_factory.cpp
@@ -2321,9 +2321,9 @@ void islot_armor::load( const JsonObject &jo )
     cata::optional<body_part_set> covers;
 
     assign_coverage_from_json( jo, "covers", covers );
-    optional( jo, false, "material_thickness", thickness, cata::nullopt );
-    optional( jo, false, "environmental_protection", env_resist, cata::nullopt );
-    optional( jo, false, "environmental_protection_with_filter", env_resist_w_filter, cata::nullopt );
+    optional( jo, was_loaded, "material_thickness", thickness, cata::nullopt );
+    optional( jo, was_loaded, "environmental_protection", env_resist, cata::nullopt );
+    optional( jo, was_loaded, "environmental_protection_with_filter", env_resist_w_filter, cata::nullopt );
 
     for( armor_portion_data &armor : sub_data ) {
         apply_optional( armor.avg_thickness, thickness );

[Saicchi]

Related or duplicate of #48364 ?