fix: use empty string path when --source flag and FUNCTION_SOURCE env var are not specified

I removed '/' from CODE_LOCATION that set the default function directory to root in commit #77. This causes this error in 1.3.0,
```
npx @google-cloud/functions-framework --target=helloWorld
The "path" argument must be of type string. Received type undefined
```

When --source flag and FUNCTION_SOURCE env var are not specified, one of the 'path' arguments in resolve() is undefined. This causes the error above.

I've added empty string as an argument in the case above. This should fix the bug.

Author: hdp617

src/index.ts

@@ -63,7 +63,7 @@ const argv = minimist(process.argv, {
 
 const CODE_LOCATION = resolve(
   process.cwd(),
-  argv[FLAG.SOURCE] || process.env[ENV.SOURCE]
+  argv[FLAG.SOURCE] || process.env[ENV.SOURCE] || ''
 );
 const PORT = argv[FLAG.PORT] || process.env[ENV.PORT] || '8080';
 const TARGET = argv[FLAG.TARGET] || process.env[ENV.TARGET] || 'function';