map_data: change sentinel value

[bates64]

The map_data format currently uses "end_data" as the sentinel string. Sentinel meaning thing that signals the end of an array. This means files can't be named "end_data", which is unlikely but not a great limitation to have. Instead, we should use an empty string "" and detect that instead.

This will require

• Changing map_data.py to use the new string
• Changing the following ASSERT condition to use the new string

papermario-dx/src/world/world.c

Line 262 in 031cfde

ASSERT_MSG(strcmp(curAsset->name, "end_data") != 0, "Asset not found: %s", assetName);