fix failing assert in connect RPC

[mvadari]

High Level Overview of Change

This PR fixes a failing assert scenario in the connect admin RPC.

Context of Change

When a node is in standalone mode and someone with admin access tries to submit this command, rippled fails an assert (and crashes in debug builds), regardless of what the other parameters are.

Example crashing command:

{
  "command": "connect"
}

Type of Change

• Bug fix (non-breaking change which fixes an issue)

API Impact

N/A, fixes something that results in a failed assert before

Before / After

Before, use of the connect RPC on a standalone node fails an assert in rippled. Now, it returns an error.

Test Plan

Tested locally.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 77.9%. Comparing base (040cd23) to head (9e03050).

Additional details and impacted files

@@           Coverage Diff           @@
##           develop   #5235   +/-   ##
=======================================
  Coverage     77.9%   77.9%           
=======================================
  Files          783     783           
  Lines        66707   66708    +1     
  Branches      8118    8115    -3     
=======================================
+ Hits         51954   51959    +5     
+ Misses       14753   14749    -4     

Files with missing lines	Coverage Δ
src/xrpld/rpc/handlers/Connect.cpp	17.6% <100.0%> (+17.6%)	⬆️

... and 2 files with indirect coverage changes