patch 9.0.1830: Vim9: crash when accessing a null object

patch 9.0.1830: Vim9: crash when accessing a null object

Problem:  Vim9: crash when accessing a null object
Solution: Check accessing a NULL object in def function

An object is NULL when the variable is declared, but the constructor
isn't called. Accessing/setting a member on the object crashed Vim.

Note: this happens inside def functions, at script level things work
differently. Accessing a NULL object member results in E1360
(correctly), while setting a value on it results in E1012 (type
mismatch) so there's still something to fix.

closes: #12973

Signed-off-by: Christian Brabandt <cb@256bit.org>
Co-authored-by: Gianmaria Bajo <mg1979.git@gmail.com>