Fix potential instability/invalidation in dlpath (#53232)

While debugging some invalidations and instabilities in code in packages
I work on, one of the issues I stumbled over is this code in this
`dlpath` method:
```julia
function dlpath(libname::Union{AbstractString, Symbol})
    handle = dlopen(libname)
    path = dlpath(handle)
    dlclose(handle)
    return path
end
```
The `dlopen` modified in this PR can in principle return `nothing`. But
there is no `dlpath` method for this. If one loads just a plain Julia,
all is fine, but under certain conditions (deep in a call chain analyzed
with Cthulhu.jl) it ended up not being able to prove that `path` will be
a string, and only inferred it as `Any`.

But if `throw_error` is set to `true` (the default, and used in the
relevant code path) then `dlopen` cannot return `nothing`, it always
returns a `String`. But the Julia compiler can't know that, as the
exception is thrown by a C helper. So instead modify the Julia code a
bit to help Julia deduce this fact by itself.

base/libdl.jl

@@ -118,7 +118,7 @@ dlopen(s::Symbol, flags::Integer = default_rtld_flags; kwargs...) =
 
 function dlopen(s::AbstractString, flags::Integer = default_rtld_flags; throw_error::Bool = true)
     ret = ccall(:jl_load_dynamic_library, Ptr{Cvoid}, (Cstring,UInt32,Cint), s, flags, Cint(throw_error))
-    if ret == C_NULL
+    if !throw_error && ret == C_NULL
         return nothing
     end
     return ret