Errors when initialising a struct with partial keywords or arguments

[yunzheng]

When initialising a struct with partial keywords or arguments the other fields become None resulting in exceptions, for example when printing the instance of the struct.

When initialising without any arguments it works and all values are 0 as expected. But when you only initialise some of the fields it breaks, example:

from dissect.cstruct import cstruct

C_DEF = """"
struct FooBar {
    uint32  foo;
    uint32  bar;
};
"""

cs = cstruct().load(C_DEF)

print(cs.FooBar())  # works as expected: <FooBar foo=0x0 bar=0x0>
print(cs.FooBar(1, 1)) # works as expected: <FooBar foo=0x1 bar=0x1>

print(cs.FooBar(1)) # breaks, bar=None
print(cs.FooBar(bar=1)) # breaks, foo=None

The exception:

$ python3 kwargs_init.py 
<FooBar foo=0x0 bar=0x0>
<FooBar foo=0x1 bar=0x1>
Traceback (most recent call last):
  File "/workspaces/dissect.cstruct/examples/kwargs_init.py", line 14, in <module>
    print(cs.FooBar(1)) # breaks, bar=None
    ^^^^^^^^^^^^^^^^^^^
  File "/workspaces/dissect.cstruct/dissect/cstruct/types/structure.py", line 388, in __repr__
    value = hex(value)
            ^^^^^^^^^^
TypeError: 'NoneType' object cannot be interpreted as an integer