Fix a number of strictNullChecks-related issues (#35795)

In preparation to work on enabling
https://www.typescriptlang.org/tsconfig/#strictNullChecks, I fixed all
the issues outside of `web_src` that came up when the option was
enabled. There was also one lint issue in web_src that apparently only
came up with the option enabled, so I fixed that as well.

`isTruthy` is introduced because Typescript has a bug regarding
`filter(Boolean)` which they are seemingly unwilling to fix.

---------

Signed-off-by: silverwind <me@silverwind.io>

web_src/js/svg.ts

@@ -181,7 +181,7 @@ export function svg(name: SvgName, size = 16, classNames?: string | string[]): s
     svgNode.setAttribute('width', String(size));
     svgNode.setAttribute('height', String(size));
   }
-  if (className) svgNode.classList.add(...className.split(/\s+/).filter(Boolean));
+  if (className) svgNode.classList.add(...className.split(/\s+/).filter(Boolean as unknown as <T>(x: T | boolean) => x is T));
   return serializeXml(svgNode);
 }