Enforce use of bool literals instead of integers

Using clang-tidy's `modernize-use-bool-literals`.
https://clang.llvm.org/extra/clang-tidy/checks/modernize-use-bool-literals.html

servers/physics_3d/joints/generic_6dof_joint_3d_sw.cpp

@@ -688,5 +688,5 @@ bool Generic6DOFJoint3DSW::get_flag(Vector3::Axis p_axis, PhysicsServer3D::G6DOF
 			break; // Can't happen, but silences warning
 	}
 
-	return 0;
+	return false;
 }