Avoid error from tsc

[tschaub]

Without this change, we get obscure failures from tsc like this:

/home/runner/work/openlayers/openlayers/node_modules/typescript/lib/tsc.js:72973
                throw e;
                ^

TypeError: Cannot read property 'flags' of undefined
    at createSymbolWithType (/home/runner/work/openlayers/openlayers/node_modules/typescript/lib/tsc.js:36787:46)
    at getUnionSignatures (/home/runner/work/openlayers/openlayers/node_modules/typescript/lib/tsc.js:30929:53)
    at resolveUnionTypeMembers (/home/runner/work/openlayers/openlayers/node_modules/typescript/lib/tsc.js:31021:34)
    at resolveStructuredTypeMembers (/home/runner/work/openlayers/openlayers/node_modules/typescript/lib/tsc.js:31330:21)
    at getSignaturesOfStructuredType (/home/runner/work/openlayers/openlayers/node_modules/typescript/lib/tsc.js:31782:32)
    at getSignaturesOfType (/home/runner/work/openlayers/openlayers/node_modules/typescript/lib/tsc.js:31788:20)
    at resolveCallExpression (/home/runner/work/openlayers/openlayers/node_modules/typescript/lib/tsc.js:42223:34)
    at resolveSignature (/home/runner/work/openlayers/openlayers/node_modules/typescript/lib/tsc.js:42494:28)
    at getResolvedSignature (/home/runner/work/openlayers/openlayers/node_modules/typescript/lib/tsc.js:42514:26)
    at checkCallExpression (/home/runner/work/openlayers/openlayers/node_modules/typescript/lib/tsc.js:42585:29)

We should try to narrow this down and file an upstream issue. But this temporarily gets things working again.

See https://github.com/openlayers/openlayers/runs/235397246 for an example failure.

[ahocevar]

Thanks!