fix contstructor to handle null options

[kkpoon]

• possible that user may give null in options parameter in constructor
• for example, no tsconfig.json file when using espower-typescript

when using espower-typescript as mocha compiler, there is an error about options parameter in constructor is null.

./node_modules/espower-typescript/node_modules/typescript-simple/index.js:29
            if (options.target == null) {
                       ^

TypeError: Cannot read property 'target' of null

after a few tests, I found that could happen when I remove the tsconfig.json file.

for evaluating undefined and null, I make this test in Typescript playground

[teppeis]

@kkpoon nice catch!