empty output cause sources && sourcesContent missing in sourcemap

esbuild version: 0.14.1
when run the following command
```sh
$ esbuild src/index.js --bundle --format=esm --outfile=dist/bundle.js --sourcemap
```
* src/index.js
```ts

function add(a,b){
return a+b;
}
```
generates the following sourcemap
```json
{
"version": 3,
"sources": [],
"sourcesContent": [],
"mappings": "",
"names": []
}

```
when input content change to
```ts
//src/index.js
function add(a,b){
return a+b;
}
console.log(add);
```
it generates
```json
{
"version": 3,
"sources": ["../src/index.js"],
"sourcesContent": ["function add(a,b){\n return a+b;\n}\nconsole.log('add:',add)"],
"mappings": ";AAAA,aAAa,GAAE,GAAE;AACf,SAAO,IAAE;AAAA;AAEX,QAAQ,IAAI,QAAO;",
"names": []
}
```

Start by reproducing the command from the issue with src/index.js in both forms and inspecting the generated sourcemap. Trace why the input containing only the add function produces empty sources and sourcesContent, then verify that the sourcemap retains the expected source information without the extra console.log call.

Written by the indexing model from the issue text.