Respect leading spaces for rendering

**Is your feature request related to a problem? Please describe.**
I'm trying to align a cursor for menu entries.

```
let cursor = if some_condition {
RichText::new("> ").color(Color32::LIGHT_GRAY).text_style(TextStyle::Monospace)
} else {
RichText::new(" ").color(Color32::LIGHT_GRAY).text_style(TextStyle::Monospace)
};

ui.label(cursor);
ui.label("Some Text");
```
Currently, the `else` condition never renders. There has to be an actual symbol besides just regular spaces. I'd prefer not to mess with the fonts as I like the default ones.

**Describe the solution you'd like**
If there is a space character it was most likely put there deliberately and thus should be rendered by default.

**Describe alternatives you've considered**
Using a custom font with some space-equivalent-yet-renderable character.

**Additional context**
I've tried looking into the source code but unfortunately couldn't figure out where the `is_empty` call is made that is causing this.

Start by tracing the ui.label rendering path and locating the is_empty call mentioned in the issue. Reproduce the example with the leading-space labels, then verify that intentional leading spaces render and the cursor and menu text align without requiring a custom font.

Written by the indexing model from the issue text.