Fix Select widget border not rendering due to BLANK name collision

[charles-001]

Summary

• Widget.BLANK (a ClassVar[bool] defaulting to False) was added in v7.1.0 to allow widgets to opt into blank rendering
• Select.BLANK is a pre-existing NoSelection sentinel object used to represent "no value selected"
• In v7.2.0, render_line and render_lines were optimized to check if self.BLANK: and skip normal rendering — but NoSelection() is truthy, so Select inadvertently enters the blank rendering path, causing its border (and all visual chrome) to disappear
• Fix: use if self.BLANK is True: to match only the boolean True, not arbitrary truthy objects

Reproduction

from textual.app import App, ComposeResult
from textual.widgets import Select

class TestApp(App):
    CSS = """
    Select { border: heavy #252e49; }
    SelectCurrent { border: none; }
    """
    def compose(self) -> ComposeResult:
        yield Select([("Option 1", 1)], prompt="Test")

TestApp().run()

Expected: Select has a heavy border
Actual (7.2.0+): Select renders with no border

Test plan

• Verified border renders correctly on 6.11.0 through 7.1.0
• Verified border disappears on 7.2.0 through 7.5.0
• Verified fix restores border rendering on 7.5.0
• Verified Widget.BLANK = True still produces blank rendering for widgets that use it