Empty Column or Row causes exception in render

[bnorm]

Example which causes exception

runMosaic {
    setContent {
        Column {}
    }
}

Exception stack trace

Exception in thread "main" java.lang.IllegalArgumentException: Row start value out of range [0,0): 0
        at com.jakewharton.mosaic.TextCanvas.get(canvas.kt:22)
        at com.jakewharton.mosaic.BoxNode.renderTo(nodes.kt:136)
        at com.jakewharton.mosaic.MosaicNode.render(nodes.kt:26)
        at com.jakewharton.mosaic.MosaicKt$runMosaic$1$2.invokeSuspend(mosaic.kt:55)
        at kotlin.coroutines.jvm.internal.BaseContinuationImpl.resumeWith(ContinuationImpl.kt:33)
        at kotlinx.coroutines.DispatchedTask.run(DispatchedTask.kt:106)
        at kotlinx.coroutines.EventLoopImplBase.processNextEvent(EventLoop.common.kt:274)
        at kotlinx.coroutines.BlockingCoroutine.joinBlocking(Builders.kt:85)
        at kotlinx.coroutines.BuildersKt__BuildersKt.runBlocking(Builders.kt:59)
        at kotlinx.coroutines.BuildersKt.runBlocking(Unknown Source)
        at kotlinx.coroutines.BuildersKt__BuildersKt.runBlocking$default(Builders.kt:38)
        at kotlinx.coroutines.BuildersKt.runBlocking$default(Unknown Source)
        at com.jakewharton.mosaic.MosaicKt.runMosaic(mosaic.kt:28)

This is actually fixed in #66 here: https://github.com/JakeWharton/mosaic/pull/66/files#diff-edb81a24f298abe6e7ff34ccd8691345abc86bed07218a3d05d49793406d9f7fR21. I could move that change into a separate PR if you aren't ready for the whole Static design yet.

[JakeWharton]

A separate PR means I can slam the merge button without much thought.

The Static stuff, on the other hand, I absolutely need time to look at and as I'm just getting home from Droidcon NYC it will be no sooner than next week (we're off Monday & Tuesday).