Bugfixes

[Diaphteiros]

How to categorize this PR?

/area quality
/kind enhancement
/priority 3

What this PR does / why we need it:
Fixes two small bugs:

1. The input formatter, which is used to build error messages for errors during templating called reflect.TypeOf(val).String(), which caused the landscaper to crash if val was nil.
2. The default value for deployitem/dataobject/target imports was an empty struct. While this probably doesn't matter when using GoTemplate (because it is text-based), this caused problems for Spiff templates without any DeployItems, because the default value was not overwritten and Spiff can't handle the empty struct when building the YAML tree. Changing the default from an empty struct to an empty map solved the problem and is also 'more correct', since all three keys hold objects (json-wise), which are represented as maps in Go. I assume that this issue went unnoticed due to Spiff being rarely used.

Release note:

Fixed a nil pointer exception that could occur during construction of the error message for a failed templating execution.

Fixed a bug which could occur when rendering exports with `Spiff` while not having any DeployItems.