Extra blank lines added when adding a property

If I add a new line to an array, recast adds unwanted additional lines and changes the original formatting.

The following code adds a property at the end of two objects and then prints them.

```
const recast = require('recast'),
types = require("ast-types");

const code1 =
`set({
prop1: 100,
prop2: [],
prop3: 100,
});`;

const code2 =
`set({
prop1: 100,
prop2: [
],
prop3: 100
});`;

const ast1 = recast.parse(code1);
const ast2 = recast.parse(code2);

const addProp = (ast, name, value) => {
const b = recast.types.builders,
properties = ast.program.body[0].expression.arguments[0].properties,
newProperty = b.property('init', b.identifier(name), b.literal(value));

properties.splice(properties.length, 0, newProperty);
};

addProp(ast1, 'prop4', 'string');
addProp(ast2, 'prop4', 'string');

console.log(recast.print(ast1).code);
console.log(recast.print(ast2).code);

```

The result is:

```
set({
prop1: 100,
prop2: [],
prop3: 100,
prop4: "string"
});
set({
prop1: 100,

prop2: [
],

prop3: 100,
prop4: "string"
});
```

while the expected result is:

```
set({
prop1: 100,
prop2: [],
prop3: 100,
prop4: "string"
});
set({
prop1: 100,
prop2: [
],
prop3: 100,
prop4: "string"
});
```

Adding a property (= adding just a line at the end of the object in this case) should not change the previous code. It could be an option if someone would need to add these lines there.

Reproduce the behavior with the supplied recast.parse and recast.print example, then trace the object-property printing path responsible for preserving blank lines. Add a regression test for both object forms and verify that appending prop4 matches the expected output without changing existing formatting.

Written by the indexing model from the issue text.