Fixes #

### Context
I was working on something else entirely and saw some easy improvements
to make in `WriteLinesToFile`.

### Changes Made
1. `File.ItemSpec` is being accessed 14 times. I think the
implementation will [make a new
string](https://github.com/jeffkl/msbuild/blob/f4e7bc9c01e75873d00a36b6f6a9e3dabb771c21/src/Build/Instance/ProjectItemInstance.cs?plain=1#L927C1-L928C1)
each time.
2. It has two code paths, overwrite vs not and both code paths were
creating the directory
3. The contents of the file to write are stored in a `StringBuilder`
with no initial capacity. I took a wild guess and set to `lines * 64`
4. If no lines are specified, a `StringBuilder` is no longer created and
instead it uses `string.Empty`
5. Since the contents are now stored in a string on all code paths, the
comparison of the current vs new is simplified since `string.Equals()`
will check the length
6. Reduced nesting by inverting the `if (File != null)` statement

### Testing
From what I can tell, the existing tests are good.

### Notes