Fix state drift for s`upsert_config` fields

[hendoxc]

What

after applying changes using upsert_config running a new plan shows it wanting to change "" -> null for some fields in upsert config

[codacy-production[bot]]

Coverage summary from Codacy

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Coverage variation	Diff coverage
:white_check_mark: +0.00% (target: -1.00%)	:white_check_mark: ∅

Coverage variation details

| | Coverable lines | Covered lines | Coverage | | ------------- | ------------- | ------------- | ------------- | | Common ancestor commit (066a2397797f68ddcf861e07c6231020d09e09c7) | 1668 | 1104 | 66.19% | | | Head commit (21d91724193e98528c79b06c8fd1917e4c930058) | 3336 (+1668) | 2208 (+1104) | 66.19% (**+0.00%**) | **Coverage variation** is the difference between the coverage for the head and common ancestor commits of the pull request branch: ` - `

Diff coverage details

| | Coverable lines | Covered lines | Diff coverage | | ------------- | ------------- | ------------- | ------------- | | Pull request (#84) | 0 | 0 | **∅ (not applicable)** | **Diff coverage** is the percentage of lines that are covered by tests out of the coverable lines that the pull request added or modified: `/ * 100%`

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