Protect from getenv("HOME") returning nullptr (#907) (#908)

* Protect from getenv("HOME") returning nullptr (e.g., in case running in systemd)

* Write proper warning in Module.cpp

Co-authored-by: Filip Leonarski <filip.leonarski@psi.ch>

slsSupportLib/include/sls/string_utils.h

@@ -20,6 +20,7 @@ Still this is better than strcpy and a buffer overflow...
 */
 template <size_t array_size>
 void strcpy_safe(char (&destination)[array_size], const char *source) {
+    assert(source != nullptr);
     assert(array_size > strlen(source));
     strncpy(destination, source, array_size - 1);
     destination[array_size - 1] = '\0';