Why do char{} and char() work as a temporary variable for a char* argument?

This is simply MSVC being behind: the rule in C++03 was that any constant expression of integer type and value 0 was a null pointer constant and could thus be converted to char*. Certainly char() qualifies—and char{} means the same thing, although it never overlapped with the rule.

Because Visual lies a lot. Especially older one. Your code prompts clang to report an error:

<source>:9:6: error: no matching function for call to 'TakePtr'

     TakePtr(char{});

     ^~~~~~~

<source>:5:6: note: candidate function not viable: no known conversion from 'char' to 'char *' for 1st argument

void TakePtr(char*); // const or not

     ^

<source>:10:6: error: no matching function for call to 'TakePtr'

     TakePtr(char());

     ^~~~~~~

<source>:5:6: note: candidate function not viable: no known conversion from 'char' to 'char *' for 1st argument

void TakePtr(char*); // const or not

     ^

2 errors generated.

Visual is known to be "wonky" in term of following C++ standard, so don't rely on it too much. Try to verify with clang / gcc, just to be sure.