NPE while printing null variables via !set

[snuyanzin]

Run any line from

!set csvDelimiter null
!set historyFile null
!set isolation null
!set numberFormat null
!set outputFormat null

then just

!set

it fails

java.lang.NullPointerException
        at sqlline.SqlLineOpts.toProperties(SqlLineOpts.java:196)
        at sqlline.Commands.config(Commands.java:487)
        at sqlline.Commands.set(Commands.java:510)
        at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke0(Native Method)
        at java.base/jdk.internal.reflect.NativeMethodAccessorImpl.invoke(Unknown Source)
        at java.base/jdk.internal.reflect.DelegatingMethodAccessorImpl.invoke(Unknown Source)
        at java.base/java.lang.reflect.Method.invoke(Unknown Source)
        at sqlline.ReflectiveCommandHandler.execute(ReflectiveCommandHandler.java:38)
        at sqlline.SqlLine.dispatch(SqlLine.java:792)
        at sqlline.SqlLine.begin(SqlLine.java:669)
        at sqlline.SqlLine.start(SqlLine.java:374)
        at sqlline.SqlLine.main(SqlLine.java:266)

[julianhyde]

Fixed in f705eea; thanks for PR #89, @snuyanzin!