Conditions on item metadata are evaluated even for empty item group

### Steps to reproduce

Project file
```xml

not P

```

I'd expect the condition to not be evaluated since `@(Empty)` is empty, so there are no items being added to `X`.

### Expected behavior

No error reported.

### Actual behavior

```
error MSB4113: Specified condition "!%(P)" evaluates to "!" instead of a boolean.
```

### Environment data
`msbuild /version` output:

Microsoft (R) Build Engine version 15.9.14-preview+gae560092f5 for .NET Framework
Copyright (C) Microsoft Corporation. All rights reserved.

15.9.14.22190