Fix backjumping by checking if broken_state.mapping is empty

src/resolvelib/resolvers.py

@@ -309,23 +309,29 @@ def _backjump(self, causes: list[RequirementInformation[RT, CT]]) -> bool:
             # Remove the state that triggered backtracking.
             del self._states[-1]
 
-            # Ensure to backtrack to a state that caused the incompatibility
-            incompatible_state = False
+            # Optimistically backtrack to a state that caused the incompatibility
             broken_state = self.state
-            while not incompatible_state:
+            while True:
                 # Retrieve the last candidate pin and known incompatibilities.
                 try:
                     broken_state = self._states.pop()
                     name, candidate = broken_state.mapping.popitem()
                 except (IndexError, KeyError):
                     raise ResolutionImpossible(causes) from None
+
+                # If the current dependencies and the incompatible dependencies
+                # are overlapping then we have found a cause of the incompatibility
                 current_dependencies = {
                     self._p.identify(d)
                     for d in self._p.get_dependencies(candidate)
                 }
-                incompatible_state = not current_dependencies.isdisjoint(
-                    incompatible_deps
-                )
+                if not current_dependencies.isdisjoint(incompatible_deps):
+                    break
+
+                # Stop backtracking if there is no state to backtrack on
+                if not broken_state.mapping:
+                    break
+
 
             incompatibilities_from_broken = [
                 (k, list(v.incompatibilities))