make evaluateExpression return falsey on not match

[jessitron]

here: https://github.com/atomist/automation-client-ts/blob/4e7a76592bb5a55d4c87fe74a9f3a292d3551bfa/src/tree/ast/FileHits.ts#L46

this returns any, which is sad. It actually returns an array of match results which is empty for no matches

We wanted to write evaluateExpression(findExportKeyword) ? "public" : "private"

so we would like evaluateAny to return MatchResult[] | null and be falsey without matches.

[timothysparg]

@jessitron I see that the signature is now evaluateExpression(pex: string | PathExpression): ExecutionResult;

ExecutionResult also comes with the companion methods from the tree-path module

export declare function isSuccessResult(a: any): a is SuccessResult;
export declare function isFailureResult(a: any): a is FailureResult;

Is the current implementation acceptable?

[ddgenome]

Yes, it seems like this is no longer relevant.