rustdoc: Print a warning if the diff when comparing to old nightlies is empty
rustdoc: Print a warning if the diff when comparing to old nightlies is empty

This avoids confusing situations where it's unclear whether there's a
bug in the diff tool or not:

```
26: @has check failed
        `XPATH PATTERN` did not match
        // @has - '//code/a[@href="{{channel}}/std/primitive.i32.html"]' 'i32'

Encountered 6 errors

------------------------------------------

info: generating a diff against nightly rustdoc

failures:
    [rustdoc] rustdoc/primitive-reexport.rs
```

aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust,graydon/rust,graydon/rust,graydon/rust,graydon/rust,aidancully/rust,graydon/rust,aidancully/rust,aidancully/rust