allow empty string for SOLANA_METRICS_CONFIG sanity checking

[jbiseda]

Problem

SOLANA_METRICS_CONFIG sanity checking fails for an empty string.

Summary of Changes

• Relax sanity check to treat an empty string the same as an unset env variable.
• Move SOLANA_METRICS_CONFIG sanity checking earlier in startup

original change for reference: sanity check metrics configuration #32799

Fixes #

[codecov]

Codecov Report

Merging #33515 (0964faf) into master (7006a6f) will decrease coverage by 0.1%.
Report is 1 commits behind head on master.
The diff coverage is 50.0%.

@@            Coverage Diff            @@
##           master   #33515     +/-   ##
=========================================
- Coverage    81.8%    81.8%   -0.1%     
=========================================
  Files         806      806             
  Lines      217458   217460      +2     
=========================================
- Hits       177987   177977     -10     
- Misses      39471    39483     +12     

[t-nelson]