gh-37629: Implement a __bool__ for posets
    
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Fixes #37547 by implementing a `__bool__` method.

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URL: #37629
Reported by: Travis Scrimshaw
Reviewer(s): Frédéric Chapoton

Author: Release Manager

src/sage/combinat/posets/posets.py

@@ -1230,6 +1230,21 @@ def unwrap(self, element):
         else:
             return element.element
 
+    def __bool__(self) -> bool:
+        r"""
+        Return if ``self`` is empty or not.
+
+        EXAMPLES::
+
+            sage: P = Poset((divisors(15), attrcall("divides")), facade=True)
+            sage: bool(P)
+            True
+            sage: P = Poset()
+            sage: bool(P)
+            False
+        """
+        return bool(self._elements)
+
     def __contains__(self, x) -> bool:
         r"""
         Return ``True`` if ``x`` is an element of the poset.