Create rule S7429: Null function pointers should not be created through transmute (#4770)

* Create rule S7429

* Update RSPEC

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Co-authored-by: yassin-kammoun-sonarsource <yassin-kammoun-sonarsource@users.noreply.github.com>
Co-authored-by: yassin-kammoun-sonarsource <yassin.kammoun@sonarsource.com>

rules/S7429/metadata.json

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+{
+}

rules/S7429/rust/metadata.json

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+{
+  "title": "Null function pointers should not be created through `transmute`",
+  "type": "BUG",
+  "status": "ready",
+  "remediation": {
+    "func": "Constant\/Issue",
+    "constantCost": "5min"
+  },
+  "tags": [
+    "clippy"
+  ],
+  "defaultSeverity": "Major",
+  "ruleSpecification": "RSPEC-7429",
+  "sqKey": "S7429",
+  "scope": "All",
+  "defaultQualityProfiles": ["Sonar way"],
+  "quickfix": "unknown",
+  "code": {
+    "impacts": {
+      "RELIABILITY": "MEDIUM"
+    },
+    "attribute": "LOGICAL"
+  }
+}

rules/S7429/rust/rule.adoc

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+== Why is this an issue?
+
+Creating a null function pointer using `transmute` results in undefined behavior, as null function pointers are not valid in Rust. Instead, use `Option<fn()>` to safely represent a nullable function pointer.
+
+=== Code examples
+
+==== Noncompliant code example
+
+[source,rust,diff-id=1,diff-type=noncompliant]
+----
+let null_fn: fn() = unsafe { std::mem::transmute(std::ptr::null::<()>()) }; // Noncompliant
+----
+
+==== Compliant solution
+
+[source,rust,diff-id=1,diff-type=compliant]
+----
+let null_fn: Option<fn()> = None; // Compliant
+----
+
+== Resources
+=== Documentation
+
+* Clippy Lints - https://rust-lang.github.io/rust-clippy/master/index.html#transmute_null_to_fn