Julia, 111,110 108 bytes

!S=minimum((r=S[i:j];'A':'Z'⊆uppercase(r) ? count(isletter,r)=>r : 2n=>"")
for i=1:(n=length(S)),j=1:n)[2]

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• returns "" for falsy cases
• very ineffective (both time and memory)
• does not optimize for window size/length - only the letter length is considered

Julia, 111,110,108 96 bytes

!S=minimum(['A':'Z'⊆(r=S[i:j])|>uppercase ? sum(isletter,r) : Inf,r]
for i=(k=keys(S)),j=k)[2]

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• credits to @MarcMush: 96 bytes
• my first attack: 108 bytes

It returns "" for falsy cases; very ineffective (both for time and for memory); it does not optimize for window size/length - only the letter length is considered

Julia, 110 bytes

r\n='A':'Z'⊆uppercase(r) ? count(isletter,r)=>r : 2n=>""
~S=minimum(S[i:j]\n for i=1:(n=length(S)),j=1:n)[2]

Attempt This Online!

• returns "" for falsy cases
• very ineffective (both time and memory)
• does not optimize for window size/length - only the letter length is considered

Julia, 111,110 108 bytes

!S=minimum((r=S[i:j];'A':'Z'⊆uppercase(r) ? count(isletter,r)=>r : 2n=>"")
for i=1:(n=length(S)),j=1:n)[2]

Attempt This Online!

• returns "" for falsy cases
• very ineffective (both time and memory)
• does not optimize for window size/length - only the letter length is considered

Julia, 111 bytes

r\n='A':'Z'⊆uppercase(r) ? count(isletter,r)=>r : n+1=>""
~S=minimum(S[i:j]\n for i=1:(n=length(S)),j=1:n)[2]

Attempt This Online!

• returns "" for falsy cases
• very ineffective (both time and memory)
• does not optimize for window size/length - only the letter length is considered
• -1 byte by changing n+1 for 2n ...

Julia, 110 bytes

r\n='A':'Z'⊆uppercase(r) ? count(isletter,r)=>r : 2n=>""
~S=minimum(S[i:j]\n for i=1:(n=length(S)),j=1:n)[2]

Attempt This Online!

• returns "" for falsy cases
• very ineffective (both time and memory)
• does not optimize for window size/length - only the letter length is considered