Null character when using range-based loop over temporary string?

When I loop over a temporary std::string (rvalue?) using a range-based for loop, there seems to be an extra character, the null terminator \0.

When the string is not temporary (lvalue instead?), there is no extra character. Why?

    std::map<char, int> m;

    for (char c : "bar") m[c] = 0;

    for (auto [c, f] : m) {
        if (c == '\0') std::cout << "this is a null char, backward slash zero" << std::endl;
        std::cout << c << std::endl; 
    }

Output:

this is a null char, backward slash zero

a
b
r

(note the empty line, where the \0 is being printed)

Compared to:

    std::map<char,int> m;
    
    std::string s = "bar";
    
    for (char c : s) m[c] = 0;    
    
    for (auto [c, f] : m) {
        if (c == '\0') std::cout << "this is a null char, backward slash zero" << std::endl;
        std::cout << c << std::endl; 
    }

Output:

a
b
r