Fix compilation of pk()-only policies into tr() descriptors

Before this fix, the added test failed with: `Unexpected("Empty Miniscript compilation")`

Author: shesek

src/policy/concrete.rs

@@ -275,8 +275,13 @@ impl<Pk: MiniscriptKey> Policy<Pk> {
                                 compilation.sanity_check()?;
                                 leaf_compilations.push((OrdF64(prob), compilation));
                             }
-                            let tap_tree = with_huffman_tree::<Pk>(leaf_compilations)?;
-                            Some(tap_tree)
+                            if !leaf_compilations.is_empty() {
+                                let tap_tree = with_huffman_tree::<Pk>(leaf_compilations)?;
+                                Some(tap_tree)
+                            } else {
+                                // no policies remaining once the extracted key is skipped
+                                None
+                            }
                         }
                     },
                 )?;
@@ -330,8 +335,14 @@ impl<Pk: MiniscriptKey> Policy<Pk> {
                                     )
                                 })
                                 .collect();
-                            let tap_tree = with_huffman_tree::<Pk>(leaf_compilations).unwrap();
-                            Some(tap_tree)
+
+                            if !leaf_compilations.is_empty() {
+                                let tap_tree = with_huffman_tree::<Pk>(leaf_compilations).unwrap();
+                                Some(tap_tree)
+                            } else {
+                                // no policies remaining once the extracted key is skipped
+                                None
+                            }
                         }
                     },
                 )?;
@@ -1119,6 +1130,14 @@ mod compiler_tests {
             .collect::<Vec<_>>();
         assert_eq!(combinations, expected_comb);
     }
+
+    #[test]
+    fn test_tr_pk_only() {
+        let policy: Policy<String> = policy_str!("pk(A)");
+        let desc = policy.compile_tr(None).unwrap();
+        // pk(A) promoted to the internal key, leaving the script tree empty
+        assert_eq!(desc.to_string(), "tr(A)#xyg3grex");
+    }
 }
 
 #[cfg(test)]