Eglot: don't use "nil" as minibuffer initial input

Eglot: don't use "nil" as minibuffer initial input

Doing M-x eglot in a buffer for which buffer-file-name is nil, prompts
the user for a major mode to manage by invoking completing-read.  The
way completing-read was called would end up with the string "nil" as
the initial minibuffer input, which is not very useful nor is it a
valid input.
* lisp/progmodes/eglot.el (eglot--guess-contact): Tweak prompt for
major mode.  (Bug#60379)

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