json files with no validKeys incompatible with rollup 0.37.0

[btakita]

ExportDefaultDeclaration expects export default to be at the beginning of the string.

https://github.com/rollup/rollup/blob/8466b4ce56415f27687842b3177138929efee62c/src/ast/nodes/ExportDefaultDeclaration.js#L56

Either we need to .trim() the string or guard against namedExports being empty causing an empty first line at:

https://github.com/rollup/rollup-plugin-json/blob/master/src/index.js#L147

[kzc]

See also: rollup/rollup#1152

[btakita]

That works for me.