babel-plugin-minify-constant-folding fails on var split = ''.split;

babel-plugin-minify-constant-folding fails on expression

```js
var split = ''.split;
split.call('', '');
```

To Reproduce

Minimal code to reproduce the bug
```js
var split = ''.split;
split.call('', '');
```
Stack Trace
```sh
Error: /node_modules/core-js/internals/indexed-object.js: don't know how to turn this value into a node
at Object.valueToNode (/Users/yuri/web/hq/node_modules/@babel/types/lib/converters/valueToNode.js:87:9)
at PluginPass.Expression (/Users/yuri/web/hq/node_modules/babel-plugin-minify-constant-folding/lib/index.js:195:26)
at PluginPass.newFn (/Users/yuri/web/hq/node_modules/@babel/traverse/lib/visitors.js:220:17)
at newFn (/Users/yuri/web/hq/node_modules/@babel/traverse/lib/visitors.js:179:21)
at NodePath._call (/Users/yuri/web/hq/node_modules/@babel/traverse/lib/path/context.js:55:20)
at NodePath.call (/Users/yuri/web/hq/node_modules/@babel/traverse/lib/path/context.js:42:17)
at NodePath.visit (/Users/yuri/web/hq/node_modules/@babel/traverse/lib/path/context.js:90:31)
at TraversalContext.visitQueue (/Users/yuri/web/hq/node_modules/@babel/traverse/lib/context.js:112:16)
at TraversalContext.visitSingle (/Users/yuri/web/hq/node_modules/@babel/traverse/lib/context.js:84:19)
at TraversalContext.visit (/Users/yuri/web/hq/node_modules/@babel/traverse/lib/context.js:140:19)
```
Configuration

How are you using babel-minify?

`babel-preset-minify trough babel API`

babel-minify version: `0.5.1`

babel version : `7.12.1`

babel-minify-config:
```js
{
builtIns: false,
deadcode: false,
// evaluate: false, // Switching babel-plugin-minify-constant-folding off solves the problem
mangle: false,
}
```