[lldb/Target] Fix `frame recognizer list` crash when registered with nullptr

One way to register a recognizer is to use RegularExpressionSP for the
module and symbol.

In order to match a symbol regardless of the module, the recognizer can
be registered with a nullptr for the module. However, this cause the
frame recognizer list command to crash because it calls
RegularExpression::GetText without checking if the shared pointer is valid.

This patch adds checks for the symbol and module RegularExpressionSP.

Differential Revision: https://reviews.llvm.org/D74212



Signed-off-by: Med Ismail Bennani <medismail.bennani@gmail.com>