llava-cli: fix base64 prompt

[Adriankhl]

The multiple image PR #6969 breaks base64 prompt processing in llava-cli

Without this change, the main function just iterate through image file paths, ignore base64 code in prompt, you have to add an empty image path --image "" as a workaround

.\bin\llava-cli.exe -m ..\..\..\models\llava-phi-3-mini-int4.gguf --mmproj ..\..\..\models\llava-phi-3-mini-mmproj-f16.gguf -c 4096 -p "<img src=\`"data:image/jpeg;base64,iVBORw0KGgoAAAANSUhEUgAAACAAAAAgCAIAAAD8GO2jAAAARklEQVR4nGLpPPSRgRSw2aaBJPVMJKkmA4xaMGrBqAWjFoxaQA3A+NnMjCQN72pkSVI/9INo1IJRC0YtGLVgRFgACAAA//8j+AbTk2+QtAAAAABJRU5ErkJggg==\`">Provide a full description" --image ""

Now this works

.\bin\llava-cli.exe -m ..\..\..\models\llava-phi-3-mini-int4.gguf --mmproj ..\..\..\models\llava-phi-3-mini-mmproj-f16.gguf -c 4096 -p "<img src=\`"data:image/jpeg;base64,iVBORw0KGgoAAAANSUhEUgAAACAAAAAgCAIAAAD8GO2jAAAARklEQVR4nGLpPPSRgRSw2aaBJPVMJKkmA4xaMGrBqAWjFoxaQA3A+NnMjCQN72pkSVI/9INo1IJRC0YtGLVgRFgACAAA//8j+AbTk2+QtAAAAABJRU5ErkJggg==\`">Provide a full description"