Adding check to bypass decoding pdbs that have no sequence points. (#19815)

Adding check to bypass decoding pdbs that have no sequence points. (#19815)

Unity has a pdb with zero sequence points somehow. If a user added a reference to the library and then attempted to set a managed breakpoint mono would hit an assert and crash when it would attempt to decode this pdb.

Call stack would look like:
```
>	mono-2.0-boehm.dll!mono_ppdb_get_seq_points(_MonoDebugMethodInfo * minfo, char * * source_file, _GPtrArray * * source_file_list, int * * source_files, MonoSymSeqPoint * * seq_points, int * n_seq_points) Line 508	C
 	mono-2.0-boehm.dll!mono_debug_get_seq_points(_MonoDebugMethodInfo * minfo, char * * source_file, _GPtrArray * * source_file_list, int * * source_files, MonoSymSeqPoint * * seq_points, int * n_seq_points) Line 1093	C
 	mono-2.0-boehm.dll!get_source_files_for_type(_MonoClass * klass) Line 6920	C
 	mono-2.0-boehm.dll!get_types_for_source_file(void * key, void * value, void * user_data) Line 7003	C
 	mono-2.0-boehm.dll!monoeg_g_hash_table_foreach(_GHashTable * hash, void(*)(void *, void *, void *) func, void * user_data) Line 364	C
 	mono-2.0-boehm.dll!mono_de_foreach_domain(void(*)(void *, void *, void *) func, void * user_data) Line 95	C
 	mono-2.0-boehm.dll!vm_commands(int command, int id, unsigned char * p, unsigned char * end, Buffer * buf) Line 7341	C
 	mono-2.0-boehm.dll!debugger_thread(void * arg) Line 10323	C
 	mono-2.0-boehm.dll!start_wrapper_internal(StartInfo * start_info, unsigned __int64 * stack_ptr) Line 1241	C
 	mono-2.0-boehm.dll!start_wrapper(void * data) Line 1315	C
 	kernel32.dll!00007ffc12017bd4()	Unknown
 	ntdll.dll!00007ffc121ece51()	Unknown
```

Adding a check to ignore pdbs like this prevents the crash and debugging can continue normally.

Related unity issue: https://issuetracker.unity3d.com/issues/macos-editor-crashes-on-mono-log-write-logfile-when-attaching-a-debugger-and-then-setting-a-breakpoint