Implement basic free and add unit test for it (#25)

## Description

Implement basic free and add unit test for it.

## Issues

<!-- use if this PR fully resolves the issue -->

Closes #23 

<!-- use if this PR is linked but should not close the issue -->

Related: #<issue-number>

## Testing

free unit test

Author: felixnagele

src/free.cpp

@@ -3,6 +3,12 @@
 namespace mem
 {
 
-void free(void* ptr) { (void)ptr; }
+void free(void* ptr)
+{
+    if (ptr == nullptr)
+    {
+        return;
+    }
+}
 
 } // namespace mem

tests/unit/test_free.cpp

@@ -0,0 +1,9 @@
+#include "malloc_from_scratch/memory_allocator.h"
+#include "test_utils.h"
+
+int main()
+{
+    mem::free(nullptr);
+
+    TEST_PASS();
+}