[alias] be explicit about empty alias sets

src/plugins/alias/abstract_state.ml

src/plugins/alias/tests/basic/oracle/assignment1.res.oracle

src/plugins/alias/tests/basic/oracle/assignment2.res.oracle

src/plugins/alias/tests/basic/oracle/assignment3.res.oracle

src/plugins/alias/tests/basic/oracle/assignment4.res.oracle

src/plugins/alias/tests/basic/oracle/assignment5.res.oracle

src/plugins/alias/tests/basic/oracle/cast1.res.oracle

src/plugins/alias/tests/basic/oracle/conditional1.res.oracle

src/plugins/alias/tests/basic/oracle/function1.res.oracle

src/plugins/alias/tests/basic/oracle/function2.res.oracle

src/plugins/alias/tests/basic/oracle/function3.res.oracle

src/plugins/alias/tests/basic/oracle/function4.res.oracle

src/plugins/alias/tests/basic/oracle/function5.res.oracle

src/plugins/alias/tests/basic/oracle/globctr.res.oracle

src/plugins/alias/tests/basic/oracle/loop.res.oracle

src/plugins/alias/tests/basic/oracle/steensgaard.res.oracle

src/plugins/alias/tests/basic/oracle/switch1.res.oracle

src/plugins/alias/tests/basic/oracle/switch2.res.oracle

src/plugins/alias/tests/basic/oracle/while_for1.res.oracle

src/plugins/alias/tests/basic/oracle/while_for2.res.oracle