Fix placement buffering error when top port has unfixable high load

[povik]

Describe the bug

This ticket is conditional on #9071 getting merged. Unfixable load on a top port now leads to the folllowing error:

[INFO RSZ-0038] Inserted 36 buffers in 81 nets.
   Iter   |    Area   | Removed | Inserted |   Pins
          |           | Buffers | Buffers  | Remaining
-------------------------------------------------------
        0 |     +0.0% |       0 |        0 |       477
       47 |     +0.0% |       0 |        0 |       430
       94 |     +0.0% |       0 |        0 |       383
      141 |     -0.2% |       5 |        3 |       336
[CRITICAL RSZ-2008] buffering pin wire155/X: wire step options empty
Command exited with non-zero status 1

Expected Behavior

An actionable warning or not issuing a warning at all

OpenROAD Environment

.

OpenLane Environment

.

To Reproduce

This can be reproduced on the I2cDeviceCtrl inner block from ihp-sg13g2/i2c-gpio-expander if one puts the below into the block's .sdc

set_load -pin_load 5 [all_inputs]
set_load -pin_load 5 [all_outputs]

Relevant log output

Screenshots

No response

Additional Context

No response

[calewis]

Also see a similar failure when setting max transition to be small