Why assume a neutral amino acid is given for acid-base reaction?
There are a few observations to make.


The amino acid only has two $\mathrm{p}K_\mathrm{a}$ values. Thus, it cannot have a side chain that is in any way acidic or basic. We only have three different possibilities: $\ce{H2AA+, HAA, AA-}$.
It was crystalline when added. Therefore, it must have been in its neutral $\ce{HAA}$ state. Otherwise, it would have been given as ‘the $\ce{HCl}$ salt …’ or ‘the $\ce{NaOH}$ salt …’


If we had any excess base in there, it would have to have been given in the question as the sodium hydroxide crystal adduct or something.