fix: add nil checks before mergo.Merge to prevent panic in gallery model installation (#7785)

Fixes #7420

Added nil checks before calling mergo.Merge in InstallModelFromGallery and InstallModel
functions to prevent panic when req.Overrides or configOverrides are nil. The panic was
occurring at models.go:248 during Qwen-Image-Edit gallery model download.

Changes:
- Added nil check for req.Overrides before merging in InstallModelFromGallery (line 126)
- Added nil check for configOverrides before merging in InstallModel (line 248)
- Added test case to verify nil configOverrides are handled without panic

Signed-off-by: majiayu000 <1835304752@qq.com>