Validation error when OpConstantNull is used to build a matrix

I get the following validation error for the following SPIR-V:

```spir-v
        %256 = OpConstantNull %v4float
        %257 = OpConstantComposite %mat3v4float %256 %256 %256
```

error:
```
OpConstantComposite Constituent <id> '256[%256]' is not a constant composite or undef.
```