fix: Honor explicitly set default value (#271)

When evaluating flags, the test harness has relied on the
[`inferDefaultFromFlag`][1] to replace an explicitly null value with an
inferred default value of the correct type.

This kind of magic only leads to confusion (hence this PR), and isn't
necessary. SDKs should be able to handle a null default as well as any
other type.

[1]:
https://github.com/launchdarkly/sdk-test-harness/blob/cb433a1206c5190796c021605c401b5cbaf5799c/sdktests/helpers.go#L137

data/data-files/server-side-eval/prerequisites.yml

@@ -316,14 +316,14 @@ evaluations:
     flagKey: flag-with-prereq-cycle-at-top-level-1
     context: { kind: "user", key: "user-key" }
     expect:
-      value: false
+      value: null
       reason: { "kind": "ERROR", "errorKind": "MALFORMED_FLAG" }
 
   - name: prerequisite cycle is detected at deeper level, recursion stops
     flagKey: flag-with-prereq-cycle-at-deeper-level-1
     context: { kind: "user", key: "user-key" }
     expect:
-      value: false
+      value: null
       reason: { "kind": "ERROR", "errorKind": "MALFORMED_FLAG" }
 
   - name: first prerequisite is a prerequisite of the second prerequisite