await throws error with null anchor

[timhall]

Love the await block, but ran into an issue with cases where an anchor isn't needed (which I think could be relatively often).

Example: https://svelte.technology/repl?version=1.44.1&gist=2280c071cccd0dbc9cf0b873540c2c4f

I think the following line needs a guard for null anchor value (needsAnchor = false), but I'm not sure what it should look like:

svelte/src/generators/dom/visitors/AwaitBlock.ts

Line 70 in 7e40ee2

${await_block}.m(${anchor}.parentNode, ${anchor});

.

[Rich-Harris]

This and #975 are fixed in 1.44.2. Thanks!