Show non-driven error on single-pin nets

Better to have both the single-pin error message and the non-driven
error message when an input pin is not connected than miss error
messages when the pin is connected to a blank net

Fixes https://gitlab.com/kicad/code/kicad/issues/10430

eeschema/erc.cpp

@@ -452,10 +452,6 @@ int ERC_TESTER::TestPinToPin()
             }
         }
 
-        // Single-pin nets are handled elsewhere
-        if( pins.size() < 2 )
-            continue;
-
         std::set<std::pair<SCH_PIN*, SCH_PIN*>> tested;
 
         SCH_PIN* needsDriver = nullptr;