bug fix deserialization of conv ops

input_zp and weight_zp attributes are disallowed for floating point inputs
for Conv2d, Conv3d, DepthwiseConv2d and TransposeConv2d

the deserialization code previously constructed operators with attributes with default values of 0 for floating point inputs.

This patch fixes this by specifying nullptr for input_zp_attr and weight_zp_attr instead.

Signed-off-by: Tai Ly <tai.ly@arm.com>
Change-Id: I37a065f9f8093363b34465ccfe9bc1e0fe424ca7