Attempt to restore getScienceDatabases() function

[oznogon]

Attempts to address #2615 by having getParentId() return 0 if the ScienceDatabase entry entity lacks a parent, and then restoring getScienceDatabases() to return a table of all unparented entry entities.

> sdb = getScienceDatabases()
> sdb[1]
entity: 0xa
> sdb[1]:getName()
Natural
> sdb[1]:getParentId()
0 -- legacy-compatible response for entry with no parent
> sdb[1]:getEntries()
table: 0x13af1270
> sdb[1]:getEntries()[1]
entity: 0xb -- first child entry of Natural
> sdb[1]:getEntries()[1]:getName()
Asteroid
> sdb[1]:getEntries()[1]:getParentId()
entity: 0xa

This also fixes hasEntries(), which attempts to call a global getEntries() instead of self:getEntries() for the count.

Before:

> getScienceDatabases()[1]:hasEntries()
api/entity/sciencedatabase.lua:119: attempt to call a nil value (global 'getEntries')
stack traceback:

After:

> getScienceDatabases()[1]:hasEntries()
true

[oznogon]

In ECS/master, it's unclear how to handle when an entity-type property like components.science_database.parent contains an empty entity. In C++, this is equivalent to an empty sp::ecs::Entity(). In Lua, these entities stringify as 0xffffffffffffffff (32-bit unsigned integer max).

In Lua, creating an empty entity such as local nothing = createEntity() still creates a valid entity with an entity ID and doesn't match the value of an empty entity. Matching by stringified entity ID entity: 0xffffffffffffffff isn't a good idea.

[oznogon]

Thanks to @GinjaNinja32, isValid() returns false for these empty entities.