patch 8.2.0155: warnings from MinGW compiler; tests fail without +float

Problem:    Warnings from MinGW compiler. (John Marriott) Json test fails when
            building without +float feature.
Solution:   Init variables. Fix Json parsing. Skip a few tests that require
            the +float feature.
diff --git a/src/vim9execute.c b/src/vim9execute.c
index a738aa1..f8efa1d 100644
--- a/src/vim9execute.c
+++ b/src/vim9execute.c
@@ -1095,6 +1095,7 @@
 	    // Computation with two float arguments
 	    case ISN_OPFLOAT:
 	    case ISN_COMPAREFLOAT:
+#ifdef FEAT_FLOAT
 		{
 		    typval_T	*tv1 = STACK_TV_BOT(-2);
 		    typval_T	*tv2 = STACK_TV_BOT(-1);
@@ -1127,6 +1128,7 @@
 		    else
 			tv1->vval.v_float = res;
 		}
+#endif
 		break;
 
 	    case ISN_COMPARELIST:
@@ -1618,7 +1620,9 @@
 				   get_var_special_name(iptr->isn_arg.number));
 		break;
 	    case ISN_PUSHF:
+#ifdef FEAT_FLOAT
 		smsg("%4d PUSHF %g", current, iptr->isn_arg.fnumber);
+#endif
 		break;
 	    case ISN_PUSHS:
 		smsg("%4d PUSHS \"%s\"", current, iptr->isn_arg.string);