SE: Concatenate string expression returns non-null string

[martin-strecker-sonarsource]

According to §12.10.5 a string concatenation never produces a null string:

These overloads of the binary + operator perform string concatenation. If an operand of string concatenation is null, an empty string is substituted. Otherwise, any non-string operand is converted to its string representation by invoking the virtual ToString method inherited from type object. If ToString returns null, an empty string is substituted.

string s = null;
s = s + null;
s.ToString(); // S2259: FP - s is not null here

Other SE rules are affected as well. Tests are added as part of #7039 in commit 09cf5fb