progress: filter out empty string

Summary:
Currently, the `render` function always return a non empty Vector, even for an empty string (shown as below), this behavior triggered a deadlock bug in `hg continue` (potentially other places as well when trying to lock a Rust mutex while holding the GIL), D43605596 fixed the bug.

```
lib/hgcommans/src/run.rs:458] &changes = [
    Text(
        "",
    ),
]
lib/hgcommands/src/run.rs:459] &last_changes = []
```
The non-empty vector makes many comparison checks fail, for example, [this logic](https://fburl.com/code/75u6gvuj) will never be executed, since `changes.is_empty()` is always `true`.

```
// Fast path: empty progress, unchanged.
if changes.is_empty() && !inner.progress_has_content {
    return Ok(());
}
```

This diff filters out empty strings from the Vector.

Reviewed By: sggutier

Differential Revision: D43611776

fbshipit-source-id: 67c07570dcd2176f05b8ab787a9f4860ea40a8c0

eden/scm/lib/progress/render/src/simple.rs

@@ -21,7 +21,12 @@ use crate::RenderingConfig;
 
 /// Render progress in to a list of termwiz changes.
 pub fn render(registry: &Registry, config: &RenderingConfig) -> Vec<Change> {
-    vec![render_string(registry, config).into()]
+    let text = render_string(registry, config);
+    if text.is_empty() {
+        vec![]
+    } else {
+        vec![text.into()]
+    }
 }
 
 /// Render progress into a multi-line string.