Fix a bug: if the less file end line is comments, the lessc command option "modify-var" will have no effect.

Here is an example: the test.less file source code:
```less
@height: 20px;
.cls {
    height: @height;
}
// comments
```
now execute the command:
```shell
bin/lessc test.less --modify-var="height=30px"
```
the expected result is the @height will transform to 30px, but it is still 20px.