Since the description of the Jim's magic missile spell (Acquisitions Incorporated, p. 76) states that it is an "improved version of" the magic missile spell, not a different spell, does the spell get nullified by shield the same way normal magic missile does?
No, it works differently
Jim's Magic Missile is not the same spell as magic missile. Just because it's an "improved version" doesn't make it the same spell. There are other spells that could be described as improvements of other spells, e.g. tidal wave and tsunami, invisibility and greater invisibility, or teleportation circle and teleport. This doesn't make them the same spell.
However, shield can still be used against Jim's Magic Missile. The part of the spell that interacts with shield is:
Make a ranged spell attack for each missile.
If you are hit by one of the attacks from Jim's Magic Missile, you can cast shield to potentially cause that attack to miss, just like any other attack. And your AC will be increased for any additional darts that are launched at you, since shield lasts until the start of your next turn.
If you like, assume that one of the "improvements" to the spell is the ability to penetrate a shield spell, although doing so is still difficult.
The spell Shield does not completely nullify Jim's Magic Missile
The spell Jim's Magic Missile from the Acquisitions Incorporated source book reads in part:
Any apprentice wizard can cast a boring old magic missile. [...] Do away with the drudgery of your grandfather's magic with this improved version of the spell, as used by Jim Darkmagic!
Make a ranged spell attack for each missile. On a hit. a missile deals 2d4 force damage to its target.
This is a unique (and improved!* ) spell that is wholly separate from the (boring* old) original version of the spell magic missile and only shares a part of its name with the original version because it references it for lore reasons. Mechanically speaking, it is a completely different spell.
Aside from the fact that it is a completely different spell from the original, Jim's version involves the caster making an attack roll for the spell's missiles. Because of this, the target may use the spell shield which
you take when you are hit by an attack.
So, not only is Jim's version not nullified by Shield outright, it may still hit the target of the attack roll is high enough.
That said, I personally think a DM has miles of leeway to Homebrew rule that Jim's Magic Missile is still nullified by Shield because Jim is good but, perhaps, not as good as he thinks.
*According to Jim, at least