[llvm-readelf] - Do not report a misleading warning when there is no string table.

[llvm-readelf] - Do not report a misleading warning when there is no string table.

This is a follow-up for D82955, which allows to continue dumping when a symbol table is broken.
When we are unable to get the string table and trying to print symbols,
the existent tool logic together with D82955 reports an error:

"st_name (0x??) is past the end of the string table of size 0x??"

Though, when there is no string table, this message becomes misleading and excessive.
It is easy to fix it though and that is what this patch does.

Differential revision: https://reviews.llvm.org/D83042