fix(navigator): ignore zero-initialized takeoff coordinates

[onelittlechildawa]

Summary

Use the current position when a standalone takeoff command leaves both coordinates at zero.

Problem

The rotary-wing takeoff path accepts (0, 0) as an explicit horizontal target.

Solution

Apply the existing current-position fallback to a zero-initialized coordinate pair.

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[github-actions]

🔎 FLASH Analysis

px4_fmu-v5x [Total VM Diff: 32 byte (0 %)]

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.0%     +32  +0.0%     +32    .text
    +0.7%     +32  +0.7%     +32    Navigator::run()
  +0.0%     +55  [ = ]       0    .debug_abbrev
  -0.0%      -2  [ = ]       0    .debug_info
  -0.0%      -5  [ = ]       0    .debug_line
  -0.3%     -32  [ = ]       0    [Unmapped]
  +0.0%     +48  +0.0%     +32    TOTAL

px4_fmu-v6x [Total VM Diff: 32 byte (0 %)]

    FILE SIZE        VM SIZE    
 --------------  -------------- 
  +0.0%     +32  +0.0%     +32    .text
    +0.7%     +32  +0.7%     +32    Navigator::run()
  +0.0%     +55  [ = ]       0    .debug_abbrev
  -0.0%      -2  [ = ]       0    .debug_info
  -0.0%      -5  [ = ]       0    .debug_line
  -0.5%     -32  [ = ]       0    [Unmapped]
  +0.0%     +48  +0.0%     +32    TOTAL

Updated: 2026-09-22T07:32:01