tflint: Fix panic when encoding empty body

[wata727]

Fixes terraform-linters/tflint-plugin-sdk#115

When encoding HCL bodies for sending to a plugin, the range of blocks is calculated from the contents of the body. However, if the body is empty, an empty range will be the return value of the HCLBodyRange func because there is no target range. Since the empty range doesn't have a filename, panic will occur when slicing source code bytes from the range:

tflint/plugin/encode.go

Line 143 in 015b01c

Config: configRange.SliceBytes(s.runner.File(configRange.Filename).Bytes),

This PR avoids this panic by always setting the filename, even if the body is empty.